mux: fix build problem

Fixes:

  In file included from drivers/serial/mux.c:37:
  include/linux/serial_core.h: In function 'uart_handle_sysrq_char':
  include/linux/serial_core.h:467: error: 'struct uart_port' has no member named 'sysrq'
  include/linux/serial_core.h:468: error: 'struct uart_port' has no member named 'sysrq'

Signed-off-by: Alexander Beregalov <a.beregalov@gmail.com>
Signed-off-by: Alan Cox <alan@lxorguk.ukuu.org.uk>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>