serial: ar933x_uart: Fix build failure with disabled console

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'

So this patch moves all the code related to console handling behind
series of CONFIG_SERIAL_AR933X_CONSOLE ifdefs.

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



Cc: Greg Kroah-Hartman <gregkh@linuxfoundation.org>
Cc: Jiri Slaby <jslaby@suse.com>
Cc: Andrey Batyiev <batyiev@gmail.com>
Reported-by: Andrey Batyiev <batyiev@gmail.com>
Tested-by: Andrey Batyiev <batyiev@gmail.com>
Signed-off-by: Petr Štetiar <ynezz@true.cz>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>