USB: ftdi_sio: clean up read completion handler

Remove superfluous error checks in completion handler:

 - No need to check private data and urb pointers as we check urb-status
   before dereferencing priv (which is not freed until urb has been killed
   on close).
 - No need to check tty as it is checked again when processing.
 - No need to check urb->number_of_packets on bulk urb.

Note that both private data and tty are checked again before processing
(possibly from work queue which also is cancelled on close).

Signed-off-by: Johan Hovold <jhovold@gmail.com>
Cc: stable <stable@kernel.org>
Signed-off-by: Greg Kroah-Hartman <gregkh@suse.de>