irtty-sir.c: Do not set_termios() on irtty_close()

Issuing set_termios() from irtty_close() causes kernel Oops for
unplugged usb-serial devices.

Since no other tty_ldisc calls set_termios() on close and no tty driver
seem to check if tty->device_data is NULL or not on entry to set_termios(),
the only solution I can come up with is to remove the irtty_stop_receiver()
call, which only updates termios.

Signed-off-by: Tommie Gannert <tommie@gannert.se>
Signed-off-by: David S. Miller <davem@davemloft.net>