usb: gadget: s3c-hsudc: use udc_start and udc_stop functions

udc_start and udc_stop reduce code duplication in comparison to
start and stop generalising calls done by all drivers
(i.e. bind and unbind) and moving these calls to common code.

Signed-off-by: Heiko Stuebner <heiko@sntech.de>
Signed-off-by: Felipe Balbi <balbi@ti.com>