Fix code that would trip -Wtype-limits

This patch fixes up all code that would throw a -Wtype-limits warning.
This sometimes involves eliminating unnecessary checks, adding a few odd
but harmless casts or just pragma'ing out the warning for a whole file
-- I tried to find the path of least resistance. I think the overall
benefit of the warning outweighs the occasional weirdness.

Change-Id: Iacd37eb1fad388d9db7267ceccb03e6dcf1ad0d2
Signed-off-by: Julius Werner <jwerner@chromium.org>
Reviewed-on: https://review.coreboot.org/c/coreboot/+/32537
Reviewed-by: Paul Menzel <paulepanter@users.sourceforge.net>
Reviewed-by: Furquan Shaikh <furquan@google.com>
Tested-by: build bot (Jenkins) <no-reply@coreboot.org>

src/drivers/pc80/rtc/mc146818rtc.c

@@ -39,6 +39,9 @@
 #define LB_CKS_LOC		0
 #endif
 
+/* Don't warn for checking >= LB_CKS_RANGE_START even though it may be 0. */
+#pragma GCC diagnostic ignored "-Wtype-limits"
+
 #include <smp/spinlock.h>
 
 #if (defined(__PRE_RAM__) &&	\