usb: chipidea: udc: fix error path in udc_start()

This patch fixes the error path of udc_start(). Now NULL is used to
unset the peripheral with otg_set_peripheral().

Cc: stable <stable@vger.kernel.org>
Reviewed-by: Richard Zhao <richard.zhao@freescale.com>
Signed-off-by: Marc Kleine-Budde <mkl@pengutronix.de>
Signed-off-by: Alexander Shishkin <alexander.shishkin@linux.intel.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>