kernel: Backport ar933x_uart build fix (fixes FS#2152)

kernel: Backport ar933x_uart build fix (fixes FS#2152)

Andrey has reported on OpenWrt's bug tracking system[1], that he
currently can't use ar93xx_uart as pure serial UART without console
(CONFIG_SERIAL_8250_CONSOLE and CONFIG_SERIAL_AR933X_CONSOLE undefined),
because compilation ends with following error:

 ar933x_uart.c: In function 'ar933x_uart_console_write':
 ar933x_uart.c:550:14: error: 'struct uart_port' has no
                               member named 'sysrq'

1. https://bugs.openwrt.org/index.php?do=details&task_id=2152

Signed-off-by: Petr Štetiar <ynezz@true.cz>