USB: ftdi_sio: unlock_kernel() on error in set_serial_info()

There was one error path where unlock_kernel() wasn't called.

This was found with a code checker (http://repo.or.cz/w/smatch.git/)
Compile tested only, sorry.

Signed-off-by: Dan Carpenter <error27@gmail.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@suse.de>