hwmon: (lm90) Re-enable interrupts after alert clears

If alert handling is broken, interrupts are disabled after an alert and
re-enabled after the alert clears. However, if there is an interrupt
handler, this does not apply if alerts were originally disabled and enabled
when the driver was loaded. In that case, interrupts will stay disabled
after an alert was handled though the alert handler even after the alert
condition clears. Address the situation by always re-enabling interrupts
after the alert condition clears if there is an interrupt handler.

Fixes: 2abdc357 ("hwmon: (lm90) Unmask hardware interrupt")
Cc: Dmitry Osipenko <digetx@gmail.com>
Signed-off-by: Guenter Roeck <linux@roeck-us.net>