usb: gadget: pxa25x_udc: 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")
this driver might return -EPROBE_DEFER if a gpio_request fails.

Cc: Eric Miao <eric.y.miao@gmail.com>
Cc: Russell King <linux@arm.linux.org.uk>
Cc: Haojian Zhuang <haojian.zhuang@gmail.com>
Cc: Felipe Balbi <balbi@ti.com>
Signed-off-by: Johan Hovold <jhovold@gmail.com>
Signed-off-by: Felipe Balbi <balbi@ti.com>