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>
diff --git a/include/uapi/linux/serial_reg.h b/include/uapi/linux/serial_reg.h
index 5db7688..dea0572 100644
--- a/include/uapi/linux/serial_reg.h
+++ b/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)