libpayload/usb: Add format string checking to usb_debug

This turns on the compiler's printf style format string checker.

BUG=b:167517417
TEST=enabled all USB controllers on volteer and fixed resulting
	compiler errors when USB_DEBUG is enabled.

Change-Id: Ic94ebcbafdde8a5f79278b5635111b99af40f892
Signed-off-by: Caveh Jalali <caveh@chromium.org>
Reviewed-on: https://review.coreboot.org/c/coreboot/+/45025
Reviewed-by: Tim Wawrzynczak <twawrzynczak@chromium.org>
Tested-by: build bot (Jenkins) <no-reply@coreboot.org>

payloads/libpayload/include/usb/usb.h

@@ -334,7 +334,7 @@ int usb_interface_check(u16 vendor, u16 device);
 #define USB_QUIRK_TEST				(1 << 31)
 #define USB_QUIRK_NONE				 0
 
-static inline void usb_debug(const char *fmt, ...)
+static inline void __attribute__((format(printf, 1, 2))) usb_debug(const char *fmt, ...)
 {
 #ifdef USB_DEBUG
 	va_list ap;