tty: xuartps: Properly guard sysrq specific code

Commit 'tty: xuartps: Implement BREAK detection, add SYSRQ support'
(0c0c47bc) introduced sysrq support
without properly guarding sysrq specific code which results in build
errors when sysrq is disabled:
	DNAME=KBUILD_STR(xilinx_uartps)" -c -o
	drivers/tty/serial/.tmp_xilinx_uartps.o
	drivers/tty/serial/xilinx_uartps.c
	drivers/tty/serial/xilinx_uartps.c: In function 'xuartps_isr':
	drivers/tty/serial/xilinx_uartps.c:247:5: error: 'struct uart_port'
	has no member named 'sysrq'
	drivers/tty/serial/xilinx_uartps.c:247:5: error: 'struct uart_port'
	has no member named 'sysrq'
	drivers/tty/serial/xilinx_uartps.c:247:5: error: 'struct uart_port'
	has no member named 'sysrq'
	make[3]: *** [drivers/tty/serial/xilinx_uartps.o] Error 1

Reported-by: Masanari Iida <standby24x7@gmail.com>
Cc: Vlad Lungu <vlad.lungu@windriver.com>
Signed-off-by: Soren Brinkmann <soren.brinkmann@xilinx.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>