pm2301: Clean-up PM2301 interrupt management

Fix the way interrupts are handled within the PM2301 charging driver.

Signed-off-by: Rajkumar Kasirajan <rajkumar.kasirajan@stericsson.com>
Signed-off-by: Lee Jones <lee.jones@linaro.org>
Reviewed-by: Olivier CLERGEAUD <olivier.clergeaud@stericsson.com>
Reviewed-by: Marcus COOPER <marcus.xm.cooper@stericsson.com>
Reviewed-by: Michel JAOUEN <michel.jaouen@stericsson.com>
Tested-by: Michel JAOUEN <michel.jaouen@stericsson.com>