drivers/timer/cavs_timer: Prevent spurious interrupts

The HDA wall clock timer is a 64 bit timer with 64 bit compare
registers, but it's being used from a 32 bit CPU.  Writing the
comparator piecewise with a 64 bit C assignment will write the low
dword first, opening the possibility that the hardware will see time
go "backwards" and trigger an interrupt incorrectly.

Disable the enable bit while setting the comparator.

Found by inspection.  In practice this will be very rare, and spurious
timer interrupts are supposed to be benign anyway (though they can
result in timeout expirations being misaligned to ticks, which might
be surprising to applications).  Best to get it right.

Signed-off-by: Andy Ross <andrew.j.ross@intel.com>

drivers/timer/cavs_timer.c

@@ -36,6 +36,9 @@ static volatile struct soc_dsp_shim_regs *shim_regs =
 
 static void set_compare(uint64_t time)
 {
+	/* Disarm the comparator to prevent spurious triggers */
+	shim_regs->dspwctcs &= ~DSP_WCT_CS_TA(TIMER);
+
 #if (TIMER == 0)
 	/* Set compare register */
 	shim_regs->dspwct0c = time;