max17042_battery: Clean up interrupt handling

Max17042 driver has dummy hardIRQ function which
does nothing but waking the threaded handler.

This patch removes the hardIRQ function and also
modifies the interrupt type to falling edge. While
we are there, change the macros to comply with the
Data sheet.

Signed-off-by: Ramakrishna Pallala <ramakrishna.pallala@intel.com>
Signed-off-by: Anton Vorontsov <anton.vorontsov@linaro.org>