USB: serial: ftdi_sio: make process-packet buffer unsigned

commit ab4cc4ef upstream.

Use an unsigned type for the process-packet buffer argument and give it
a more apt name.

Reviewed-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>
Signed-off-by: Johan Hovold <johan@kernel.org>
Signed-off-by: Paul Gortmaker <paul.gortmaker@windriver.com>