ftdi_sio: fix kref leak

Commit 4a90f09b added kref stuff to
ftdi_sio, but missed tty_kref_put at one exit point in
ftdi_process_read.

Signed-off-by: Jim Paris <jim@jtan.com>
Signed-off-by: Alan Cox <alan@redhat.com>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>