usb: dwc2/gadget: delay enabling irq once hardware is configured properly

This patch fixes kernel panic/interrupt storm/etc issues if bootloader
left s3c-hsotg module in enabled state. Now interrupt handler is enabled
only after proper configuration of hardware registers.

Signed-off-by: Marek Szyprowski <m.szyprowski@samsung.com>
Signed-off-by: Robert Baldyga <r.baldyga@samsung.com>
Cc: stable <stable@vger.kernel.org> # 3.16
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>