tty: omap-serial: remove set but unused variable

Fix the following warning:
drivers/tty/serial/omap-serial.c: In function serial_omap_rlsi:
drivers/tty/serial/omap-serial.c:496:16: warning: variable ch set but not used [-Wunused-but-set-variable]

The character read is useless according to the table 23-246 of the omap4
TRM. So we can drop it.
Reported-by: NHulk Robot <hulkci@huawei.com>
Signed-off-by: NXiongfeng Wang <wangxiongfeng2@huawei.com>
Link: https://lore.kernel.org/r/1575617863-32484-1-git-send-email-wangxiongfeng2@huawei.comSigned-off-by: NGreg Kroah-Hartman <gregkh@linuxfoundation.org>

drivers/tty/serial/omap-serial.c

@@ -493,10 +493,13 @@ static unsigned int check_modem_status(struct uart_omap_port *up)
 static void serial_omap_rlsi(struct uart_omap_port *up, unsigned int lsr)
 {
 	unsigned int flag;
-	unsigned char ch = 0;
 
+	/*
+	 * Read one data character out to avoid stalling the receiver according
+	 * to the table 23-246 of the omap4 TRM.
+	 */
 	if (likely(lsr & UART_LSR_DR))
-		ch = serial_in(up, UART_RX);
+		serial_in(up, UART_RX);
 
 	up->port.icount.rx++;
 	flag = TTY_NORMAL;