Staging: Fix leak in drivers/staging/at76_usb.c

Fix leak in at76_usb as reported in:
http://bugzilla.kernel.org/show_bug.cgi?id=11778




Reported-by: Daniel Marjamäki <danielm77@spray.se>
Signed-off-by: Diego Calleja <diegocg@gmail.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@suse.de>