serial: sirf: fix the issue that HW flow control doesn't work for BT

>From HW spec, when rxfifo's data is less than AFC_RX_THD(RX threshhold), RTS
signal is active. otherwise, RTS signal is inactive.

Crrently the RX threshhold is set as zero, so RTS has no chance to be
active. This patch replaces the default 0 by a positive number.

Signed-off-by: Qipan Li <Qipan.Li@csr.com>
Signed-off-by: Barry Song <Baohua.Song@csr.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>

drivers/tty/serial/sirfsoc_uart.c

@@ -344,7 +344,8 @@ static void sirfsoc_uart_enable_ms(struct uart_port *port)
 	if (sirfport->uart_reg->uart_type == SIRF_REAL_UART) {
 		wr_regl(port, ureg->sirfsoc_afc_ctrl,
 				rd_regl(port, ureg->sirfsoc_afc_ctrl) |
-				SIRFUART_AFC_TX_EN | SIRFUART_AFC_RX_EN);
+				SIRFUART_AFC_TX_EN | SIRFUART_AFC_RX_EN |
+				SIRFUART_AFC_CTRL_RX_THD);
 		if (!sirfport->is_atlas7)
 			wr_regl(port, ureg->sirfsoc_int_en_reg,
 					rd_regl(port, ureg->sirfsoc_int_en_reg)