USB: serial: ftdi_sio: clean up receive processing

[ Upstream commit ce054039ba5e47b75a3be02a00274e52b06a6456 ]

Clean up receive processing by dropping the character pointer and
keeping the length argument unchanged throughout the function.

Also make it more apparent that sysrq processing can consume a
characters by adding an explicit continue.

Reviewed-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>
Signed-off-by: Johan Hovold <johan@kernel.org>
Signed-off-by: Sasha Levin <sashal@kernel.org>