serial: xilinx_uartps: Make cdns_rs485_supported static

The sparse tool complains as follows:

drivers/tty/serial/xilinx_uartps.c:222:21: warning:
	symbol 'cdns_rs485_supported' was not declared. Should it be static?

This symbol is not used outside xilinx_uartps.c, so marks it static.
Signed-off-by: Jinjie Ruan <ruanjinjie@huawei.com>
Link: https://lore.kernel.org/r/20240819120107.3884973-1-ruanjinjie@huawei.comSigned-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>

drivers/tty/serial/xilinx_uartps.c

@@ -219,7 +219,7 @@ struct cdns_platform_data {
 	u32 quirks;
 };
 
-struct serial_rs485 cdns_rs485_supported = {
+static struct serial_rs485 cdns_rs485_supported = {
 	.flags = SER_RS485_ENABLED | SER_RS485_RTS_ON_SEND |
 		 SER_RS485_RTS_AFTER_SEND,
 	.delay_rts_before_send = 1,