src: Fix the warning "type 'hex' are always defined"

This is spotted using "./util/lint/kconfig_lint"
While at it, do the check in C and not the preprocessor.

Change-Id: Icfda267936a23d9d14832116d67571f42f685906
Signed-off-by: Elyes HAOUAS <ehaouas@noos.fr>
Reviewed-on: https://review.coreboot.org/c/31050
Tested-by: build bot (Jenkins) <no-reply@coreboot.org>
Reviewed-by: Nico Huber <nico.h@gmx.de>

src/device/pci_ops_mmconf.c

@@ -15,7 +15,7 @@
 #include <device/pci.h>
 #include <device/pci_ops.h>
 
-#if !defined(CONFIG_MMCONF_BASE_ADDRESS) || !CONFIG_MMCONF_BASE_ADDRESS
+#if (CONFIG_MMCONF_BASE_ADDRESS == 0)
 #error "CONFIG_MMCONF_BASE_ADDRESS needs to be non-zero!"
 #endif