udc: dfu: s3c_udc: Provide function to check if USB reset was asserted

New dfu_usb_get_reset() method is necessary to distinct two different
use cases of dfu-util program.
This method checks if the USB bus reset has been really performed after
DFU DETACH.

Without this function the previous DFU behavior is preserved.

Signed-off-by: Lukasz Majewski <l.majewski@samsung.com>