ab8500-charger: Check return value of regulator_enable

Check the return value of regulator_enable to silence the following
type of warnings:
drivers/power/ab8500_charger.c:1390:20: warning: ignoring return value
of ‘regulator_enable’, declared with attribute warn_unused_result
[-Wunused-result]

Signed-off-by: Sachin Kamat <sachin.kamat@linaro.org>
Acked-by: Lee Jones <lee.jones@linaro.org>
Signed-off-by: Anton Vorontsov <anton@enomsg.org>