mxser: remove tty_port_tty_get from mxser_check_modem_status

mxser_check_modem_status is called with tty parameter, so the
reference should be increased by callers already -- for ioctl
syscall it is held whole time gap since open to close, for
interrupt, the reference count is increased in the irq handler.

There is no tty_kref_put in that function, so this also fixes
a refcounting bug.

Signed-off-by: Jiri Slaby <jirislaby@gmail.com>
Signed-off-by: Alan Cox <alan@lxorguk.ukuu.org.uk>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>