rtc: remove "RTC_ALM_SET mode" bugs

This fixes a common glitch in how RTC drivers handle two "set alarm" modes,
by getting rid of the surprising/hidden one that was rarely implemented
correctly (and which could expose nonportable hardware-specific behavior).

The glitch comes from the /dev/rtcX logic implementing the legacy
RTC_ALM_SET (limited to 24 hours, needing RTC_AIE_ON) ioctl on top of the
RTC driver call providing access to the newer RTC_WKALM_SET (without those
limitations) by initializing the day/month/year fields to be invalid ...
that second mode.

Now, since few RTC drivers check those fields, and most hardware misbehaves
when faced with invalid date fields, many RTC drivers will set bogus alarm
times on those RTC_ALM_SET code paths.  (Several in-tree drivers have that
issue, and I also noticed it with code reviews on several new RTC drivers.)

This patch ensures that RTC drivers never see such invalid alarm fields, by
moving some logic out of rtc-omap into the RTC_ALM_SET code and adding an
explicit check (which will prevent the issue on other code paths).

Signed-off-by: David Brownell <dbrownell@users.sourceforge.net>
Cc: Scott Wood <scottwood@freescale.com>
Cc: Alessandro Zummo <a.zummo@towertech.it>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>

Documentation/rtc.txt

@@ -147,7 +147,7 @@ RTC class framework, but can't be supported by the older driver.
 
     *	RTC_AIE_ON, RTC_AIE_OFF, RTC_ALM_SET, RTC_ALM_READ ... when the RTC
 	is connected to an IRQ line, it can often issue an alarm IRQ up to
-	24 hours in the future.
+	24 hours in the future.  (Use RTC_WKALM_* by preference.)
 
     *	RTC_WKALM_SET, RTC_WKALM_RD ... RTCs that can issue alarms beyond
 	the next 24 hours use a slightly more powerful API, which supports
@@ -175,10 +175,7 @@ driver returns ENOIOCTLCMD.  Some common examples:
 	called with appropriate values.
 
     *	RTC_ALM_SET, RTC_ALM_READ, RTC_WKALM_SET, RTC_WKALM_RD: the
-	set_alarm/read_alarm functions will be called.  To differentiate
-	between the ALM and WKALM, check the larger fields of the rtc_wkalrm
-	struct (like tm_year).  These will be set to -1 when using ALM and
-	will be set to proper values when using WKALM.
+	set_alarm/read_alarm functions will be called.
 
     *	RTC_IRQP_SET, RTC_IRQP_READ: the irq_set_freq function will be called
 	to set the frequency while the framework will handle the read for you