firmware/dmi_scan: Uninline dmi_get_bios_year() helper

Uninline dmi_get_bios_year() which, in particular, allows us
to optimize it in the future.

While doing this, convert the function to return an error code
when BIOS date is not present or not parsable, or CONFIG_DMI=n.

Additionally, during the move, add a bit of documentation.

Suggested-by: Bjorn Helgaas <helgaas@kernel.org>
Suggested-by: Rafael J. Wysocki <rafael@kernel.org>
Signed-off-by: Andy Shevchenko <andriy.shevchenko@linux.intel.com>
Reviewed-by: Jean Delvare <jdelvare@suse.de>
Reviewed-by: Rafael J. Wysocki <rafael.j.wysocki@intel.com>
Acked-by: Thomas Gleixner <tglx@linutronix.de>
Cc: Bjorn Helgaas <bhelgaas@google.com>
Cc: Linus Torvalds <torvalds@linux-foundation.org>
Cc: Lukas Wunner <lukas@wunner.de>
Cc: Peter Zijlstra <peterz@infradead.org>
Cc: Rafael J . Wysocki <rjw@rjwysocki.net>
Cc: linux-acpi@vger.kernel.org
Cc: linux-pci@vger.kernel.org
Fixes: 492a1abd61 ("dmi: Introduce the dmi_get_bios_year() helper function")
Signed-off-by: Ingo Molnar <mingo@kernel.org>

drivers/firmware/dmi_scan.c

@@ -1004,6 +1004,26 @@ out:
 }
 EXPORT_SYMBOL(dmi_get_date);
 
+/**
+ *	dmi_get_bios_year - get a year out of DMI_BIOS_DATE field
+ *
+ *	Returns year on success, -ENXIO if DMI is not selected,
+ *	or a different negative error code if DMI field is not present
+ *	or not parseable.
+ */
+int dmi_get_bios_year(void)
+{
+	bool exists;
+	int year;
+
+	exists = dmi_get_date(DMI_BIOS_DATE, &year, NULL, NULL);
+	if (!exists)
+		return -ENODATA;
+
+	return year ? year : -ERANGE;
+}
+EXPORT_SYMBOL(dmi_get_bios_year);
+
 /**
  *	dmi_walk - Walk the DMI table and get called back for every record
  *	@decode: Callback function