include/console/uart: make index parameter unsigned

The UART index is never negative, so make it unsigned and drop the
checks for the index to be non-negative.

Change-Id: I64bd60bd2a3b82552cb3ac6524792b9ac6c09a94
Signed-off-by: Felix Held <felix-coreboot@felixheld.de>
Reviewed-on: https://review.coreboot.org/c/coreboot/+/45294
Tested-by: build bot (Jenkins) <no-reply@coreboot.org>
Reviewed-by: Marshall Dawson <marshalldawson3rd@gmail.com>
Reviewed-by: Aaron Durbin <adurbin@chromium.org>
Reviewed-by: Furquan Shaikh <furquan@google.com>

src/soc/sifive/fu540/uart.c

@@ -6,7 +6,7 @@
 #include <soc/addressmap.h>
 #include <soc/clock.h>
 
-uintptr_t uart_platform_base(int idx)
+uintptr_t uart_platform_base(unsigned int idx)
 {
 	if (idx < 2)
 		return FU540_UART(idx);