power: supply: mp2629_charger: use platform_get_irq()

Calling platform_get_irq_optional() doesn't make sense if you then bail out
on any error it returns.  Switch to calling platform_get_irq() instead and
remove dev_err() call as platform_get_irq() already curses loudly on error.

Signed-off-by: Sergey Shtylyov <s.shtylyov@omp.ru>
Signed-off-by: Sebastian Reichel <sebastian.reichel@collabora.com>