usb: phy: gpio-vbus: fix deferred probe from __init

Move probe out of __init section and don't use platform_driver_probe
which cannot be used with deferred probing.

Since commit e9354576 ("gpiolib: Defer failed gpio requests by default")
and 04bf3011

 ("regulator: Support driver probe deferral") this driver
might return -EPROBE_DEFER if a gpio_request or regulator_get fails.

Cc: Felipe Balbi <balbi@ti.com>
Signed-off-by: Johan Hovold <jhovold@gmail.com>
Signed-off-by: Felipe Balbi <balbi@ti.com>