[MIPS] Misc fixes for plat_irq_dispatch functions

o adds missing ST0_IM masks, which caused the logging of valid interrupts
   as spurious
 o stops pnx8550 to log every interrupt as spurious
 o adds cause register masks for ip22/ip32, which caused handling of masked
   interrupts
 o removes some superfluous parentheses in the SNI interrupt code

Signed-Off-By: Thiemo Seufer <ths@networkno.de>
Signed-off-by: Ralf Baechle <ralf@linux-mips.org>

arch/mips/sgi-ip22/ip22-int.c

@@ -237,7 +237,7 @@ extern void indy_8254timer_irq(void);
 
 asmlinkage void plat_irq_dispatch(void)
 {
-	unsigned int pending = read_c0_cause();
+	unsigned int pending = read_c0_status() & read_c0_cause();
 
 	/*
 	 * First we check for r4k counter/timer IRQ.