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

[ Upstream commit ab4cc4ef6724ea588e835fc1e764c4b4407a70b7 ]

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: Sasha Levin <sashal@kernel.org>