Update to v4.15 kernel headers.

Test: NA.
Change-Id: Ic06b0506705a46ed11e3b9f212515d34147c8135
diff --git a/original/uapi/linux/serial_reg.h b/original/uapi/linux/serial_reg.h
index 619fe61..be07b54 100644
--- a/original/uapi/linux/serial_reg.h
+++ b/original/uapi/linux/serial_reg.h
@@ -158,6 +158,7 @@
  */
 #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)