pm2301-charger: Check return value of regulator_enable

Check the return value of regulator_enable to silence the following
warning:
drivers/power/pm2301_charger.c:725:20: 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>