twl4030-charger: Fix compiler warning with regulator_enable()

The return value of regulator_enable need to be checked. This patch fixes
the following warning:
drivers/power/twl4030_charger.c: In function ‘twl4030_charger_enable_usb’:
drivers/power/twl4030_charger.c:192:20: warning: ignoring return value of ‘regulator_enable’, declared with attribute warn_unused_result [-Wunused-result]

Signed-off-by: Peter Ujfalusi <peter.ujfalusi@ti.com>
Signed-off-by: Anton Vorontsov <anton@enomsg.org>