rtc: abx80x: Disable alarm feature if no interrupt attached

Commit 795cda8338ea ("rtc: interface: Fix long-standing race when setting
alarm") exposed an issue where the rtc-abx80x driver does not clear the
alarm feature bit, but instead relies on the set_alarm operation to return
invalid.

For example, when a RTC_UIE_ON ioctl is handled, it should abort at the
feature validation. Instead, it proceeds to the rtc_timer_enqueue(),
which used to return an error from the set_alarm call. However,
following the race condition handling, which likely should not be
discarding predecing errors, a success condition is returned to the
ioctl() caller. This results in (for example):
hwclock: select() to /dev/rtc0 to wait for clock tick timed out

Notwithstanding the validity of the race condition handling, if an interrupt
wasn't specified, or could not be attached, the driver should clear the
alarm feature bit.

Fixes: 718a820a303c ("rtc: abx80x: add alarm support")
Signed-off-by: Anthony Pighin <anthony.pighin@nokia.com>
Link: https://patch.msgid.link/BN0PR08MB69510928028C933749F4139383D1A@BN0PR08MB6951.namprd08.prod.outlook.com
Signed-off-by: Alexandre Belloni <alexandre.belloni@bootlin.com>