stm32/hwtimer32: Check if interrupts are disabled when setting timestamp
stm32/hwtimer32: Check if interrupts are disabled when setting timestamp

CL:3347658 introduces disabling interrupts when setting new timestamp to
the timer. To make sure that __hw_clock_source_set() won't be used with
enabled interrupts let's add ASSERT which will check that.

This patch also adds explaination, why clearing all status bits in the
timer is correct.

BUG=b:200828093
BRANCH=none
TEST=Compile EC with CONFIG_CMD_FORCETIME enabled. Use 'forcetime'
     command to set time, then use 'timerinfo' to check if correct
     time is set.

Signed-off-by: Patryk Duda <6aed17a362577cf10cb5675888c1635d522769c1@semihalf.com>
Change-Id: I2945edbe71e4e9991b4e8eaa462d8c1d08a22d23
Reviewed-on: https://chromium-review.googlesource.com/c/chromiumos/platform/ec/+/3395878
Reviewed-by: Tom Hughes <fc5c12f30a5ed2baa0581f44ee14ad1eb7b20d83@chromium.org>

coreboot/chrome-ec,coreboot/chrome-ec,coreboot/chrome-ec,coreboot/chrome-ec,coreboot/chrome-ec,coreboot/chrome-ec