usb: chipidea: usbmisc: unset global varibale usbmisc on driver remove

The probe function checks usbmisc to be NULL in the beginning. Without
this patch the can only be loaded once.

Signed-off-by: Marc Kleine-Budde <mkl@pengutronix.de>
Signed-off-by: Michael Grzeschik <m.grzeschik@pengutronix.de>
Signed-off-by: Alexander Shishkin <alexander.shishkin@linux.intel.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>