serial: Add define for max baud rate divisor

Add a define for the maximum baud rate divisor, to improve code
readability.
Signed-off-by: Ed Blake <ed.blake@sondrel.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>

drivers/tty/serial/8250/8250_mtk.c

@@ -61,7 +61,7 @@ mtk8250_set_termios(struct uart_port *port, struct ktermios *termios,
 	 * registers to their default values.
 	 */
 	baud = uart_get_baud_rate(port, termios, old,
-				  port->uartclk / 16 / 0xffff,
+				  port->uartclk / 16 / UART_DIV_MAX,
 				  port->uartclk);
 
 	if (baud <= 115200) {

drivers/tty/serial/8250/8250_omap.c

@@ -199,7 +199,7 @@ static void omap_8250_get_divisor(struct uart_port *port, unsigned int baud,
 	 * Old custom speed handling.
 	 */
 	if (baud == 38400 && (port->flags & UPF_SPD_MASK) == UPF_SPD_CUST) {
-		priv->quot = port->custom_divisor & 0xffff;
+		priv->quot = port->custom_divisor & UART_DIV_MAX;
 		/*
 		 * I assume that nobody is using this. But hey, if somebody
 		 * would like to specify the divisor _and_ the mode then the
@@ -358,7 +358,7 @@ static void omap_8250_set_termios(struct uart_port *port,
 	 * Ask the core to calculate the divisor for us.
 	 */
 	baud = uart_get_baud_rate(port, termios, old,
-				  port->uartclk / 16 / 0xffff,
+				  port->uartclk / 16 / UART_DIV_MAX,
 				  port->uartclk / 13);
 	omap_8250_get_divisor(port, baud, priv);
 

drivers/tty/serial/8250/8250_port.c

@@ -2601,7 +2601,7 @@ static unsigned int serial8250_get_baud_rate(struct uart_port *port,
 	 * causing transmission errors.
 	 */
 	return uart_get_baud_rate(port, termios, old,
-				  port->uartclk / 16 / 0xffff,
+				  port->uartclk / 16 / UART_DIV_MAX,
 				  port->uartclk);
 }
 

include/uapi/linux/serial_reg.h

@@ -157,6 +157,7 @@
  */
 #define UART_DLL	0	/* Out: Divisor Latch Low */
 #define UART_DLM	1	/* Out: Divisor Latch High */
+#define UART_DIV_MAX	0xFFFF	/* Max divisor value */
 
 /*
  * LCR=0xBF (or DLAB=1 for 16C660)