Reduce warnings/errors in libpayload when using picky compiler options

The new build system uses quite a few more -W flags for the compiler by
default than the old one. And that's for the better.

Change-Id: Ia8e3d28fb35c56760c2bd0983046c7067e8c5dd6
Signed-off-by: Patrick Georgi <patrick@georgi-clan.de>
Reviewed-on: http://review.coreboot.org/72
Tested-by: build bot (Jenkins)
Reviewed-by: Uwe Hermann <uwe@hermann-uwe.de>

payloads/libpayload/drivers/usb/uhci.c

@@ -65,7 +65,7 @@ static void
 td_dump (td_t *td)
 {
 	char td_value[3];
-	char *td_type;
+	const char *td_type;
 	switch (td->pid) {
 		case UHCI_SETUP:
 			td_type="SETUP";