serdev: ttyport: call tiocmget and tiocmset ops directly

The tty struct holds a pointer to the driver's tty operations so drop
the unnecessary driver dereference when calling tiocmget and tiocmset.

Note that this also makes the calls match the preceding sanity checks as
expected.

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