soc/intel/fast_spi/Makefile: Fix exit of 16mib-check recipe

Currently, if everything worked fine, `$fail` will be unset, leading
to the following `if` statement:

    if [ -eq 1 ]

Resulting in the error message:

    /bin/sh: line 9: [: -eq: unary operator expected

Fix this by removing the whole `if`, we can just use `exit`.

Change-Id: I1bc7508d2a45a2bec07ef46b9c5d9d0b740fbc74
Signed-off-by: Nico Huber <nico.h@gmx.de>
Reviewed-on: https://review.coreboot.org/c/coreboot/+/51324
Tested-by: build bot (Jenkins) <no-reply@coreboot.org>
Reviewed-by: Angel Pons <th3fanbus@gmail.com>
Reviewed-by: Furquan Shaikh <furquan@google.com>

src/soc/intel/common/block/fast_spi/Makefile.inc

@@ -44,7 +44,7 @@ $(call add_intermediate, check-fmap-16mib-crossing, $(obj)/fmap_config.h)
 		    end=$$((start+size-1)); \
 		    if [ $$start -lt 16777216 ] && [ $$end -ge 16777216 ]; then echo "ERROR:" $$x "crosses 16MiB boundary"; fail=1; break; fi; \
 		done; \
-		if [ $$fail -eq 1 ]; then false; fi
+		exit $$fail
 
 CBFSTOOL_ADD_CMD_OPTIONS += --ext-win-base $(CONFIG_EXT_BIOS_WIN_BASE) --ext-win-size $(CONFIG_EXT_BIOS_WIN_SIZE)