[PATCH] USB: gadget-serial: fix a deadlock when closing the serial device

When closing the device, the driver acquires/release twice the
port lock before/after waiting for the data to be completely
sent. Therefore it will dead lock.

This patch fixes it and also uses the generic scheduler services
for waiting for an event.

Signed-off-by: Franck Bui-Huu <vagabon.xyz@gmail.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@suse.de>