staging: gdm7240: fix memory leak on failure path

init_usb() may fail after some of mux_rxes already allocated.
So we need to release them on the failure path.

Found by Linux Driver Verification project (linuxtesting.org).

Signed-off-by: Alexey Khoroshilov <khoroshilov@ispras.ru>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>