serial: Update NS16550_t and struct NS16550

Typedefs should not be used in U-Boot and structs should be lower case.
Update the code to use struct ns16550 consistently.

Put a header guard on the file while we are here.

Signed-off-by: Simon Glass <sjg@chromium.org>

Reviewed-by: Andy Shevchenko <andriy.shevchenko@linux.intel.com>
diff --git a/board/Synology/ds109/ds109.c b/board/Synology/ds109/ds109.c
index aa2987d..c36e4d3 100644
--- a/board/Synology/ds109/ds109.c
+++ b/board/Synology/ds109/ds109.c
@@ -106,10 +106,10 @@
 	printf("Synology reset...");
 	udelay(50000);
 
-	b_d = ns16550_calc_divisor((NS16550_t)CONFIG_SYS_NS16550_COM2,
-		CONFIG_SYS_NS16550_CLK, 9600);
-	NS16550_init((NS16550_t)CONFIG_SYS_NS16550_COM2, b_d);
-	NS16550_putc((NS16550_t)CONFIG_SYS_NS16550_COM2, SOFTWARE_REBOOT);
+	b_d = ns16550_calc_divisor((struct ns16550 *)CONFIG_SYS_NS16550_COM2,
+				   CONFIG_SYS_NS16550_CLK, 9600);
+	NS16550_init((struct ns16550 *)CONFIG_SYS_NS16550_COM2, b_d);
+	NS16550_putc((struct ns16550 *)CONFIG_SYS_NS16550_COM2, SOFTWARE_REBOOT);
 }
 
 /* Support old kernels */