usb-gadget: unlock data->lock mutex on error path in ep_write()

ep_write() acquires data->lock mutex in get_ready_ep() and releases it
on all paths except for one: when usb_endpoint_xfer_isoc() failed. The
patch adds mutex_unlock(&data->lock) at that path.

It is similar to commit 00cc7a5f ("usb-gadget: unlock data->lock mutex on error path in ep_read()"),
it was not fixed at that time by accident.

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

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