cpu/x86/smm/smm_module_loader: Compare num_concurrent_stacks to size_t

Spotted out using -Wconversion gcc warning option.

Change-Id: I11e4792804f0f7b5a7ce504c46654c1bff775c32
Signed-off-by: Elyes HAOUAS <ehaouas@noos.fr>
Reviewed-on: https://review.coreboot.org/c/coreboot/+/33804
Tested-by: build bot (Jenkins) <no-reply@coreboot.org>
Reviewed-by: Angel Pons <th3fanbus@gmail.com>
Reviewed-by: Paul Menzel <paulepanter@users.sourceforge.net>

src/cpu/x86/smm/smm_module_loader.c

@@ -185,7 +185,7 @@ static int smm_module_setup_stub(void *smbase, struct smm_loader_params *params,
 	void *stacks_top;
 	size_t size;
 	char *base;
-	int i;
+	size_t i;
 	struct smm_stub_params *stub_params;
 	struct rmodule smm_stub;