usb: phy: gpio-vbus: don't ignore regulator APIs return value

Due to recent changes to regulator API, all
users which don't check regulator_{en,dis}able()'s
return value will generate compile warnings.

Add such checks to gpio-vbus.

Cc: Mark Brown <broonie@opensource.wolfsonmicro.com>
Signed-off-by: Felipe Balbi <balbi@ti.com>