serqt: initial clean up pass for tty side

Avoid using port->tty where possible (makes refcount fixing easier
later).

Remove unused code (the ioctl path is not used if the device has
mget/mset functions)

Remove various un-needed typecasts and long names so it could read it to
do the changes.
Signed-off-by: NAlan Cox <alan@lxorguk.ukuu.org.uk>
Signed-off-by: NLinus Torvalds <torvalds@linux-foundation.org>