hwmon: (lm90) Unmask hardware interrupt

The ALERT interrupt is enabled by default after power-on, but it could
be masked by bootloader. For example this is the case on Acer A500 tablet
device. Unmask the hardware interrupt if interrupt is provided.

Signed-off-by: Dmitry Osipenko <digetx@gmail.com>
Signed-off-by: Guenter Roeck <linux@roeck-us.net>