tty: make tty_change_softcar() more understandable

* rename 'arg' to 'enable' as that is what it means.
* make 'bit' a tcflag_t, not int, as that is what cflags are.

Signed-off-by: "Jiri Slaby (SUSE)" <jirislaby@kernel.org>
Link: https://lore.kernel.org/r/20230810091510.13006-9-jirislaby@kernel.org


Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>