usb: dwc2: remove non-functional clock gating

During typical gadget operation, dwc2 clock was enabled 3 times: from
dwc2_gadget_init(), dwc2_hsotg_udc_start() and dwc2_hsotg_pullup(), and
then disabled in s3c_hsotg_pullup(), s3c_hsotg_udc_stop() and
dwc2_hsotg_remove(). This really makes no sense, so leave clock control
code only in dwc2_gadget_init/remove functions.

Tested-by: John Youn <johnyoun@synopsys.com>
Acked-by: John Youn <johnyoun@synopsys.com>
Signed-off-by: Marek Szyprowski <m.szyprowski@samsung.com>
Signed-off-by: Felipe Balbi <balbi@ti.com>