region: add rdev_chain_full()

Instead of open coding an offset of 0 and querying the size of a
region device provide a rdev_chain_full() helper function that
does that for the caller. For the existing users that match this pattern
convert them to using rdev_chain_full().

Change-Id: Ie316790a8a5b16a7f7e22f86f58bd2e633c19450
Signed-off-by: Aaron Durbin <adurbin@chromium.org>
Reviewed-on: https://review.coreboot.org/c/coreboot/+/36683
Reviewed-by: Julius Werner <jwerner@chromium.org>
Tested-by: build bot (Jenkins) <no-reply@coreboot.org>

src/lib/fmap.c

@@ -49,8 +49,7 @@ static int find_fmap_directory(struct region_device *fmrd)
 
 		cache = car_get_var_ptr(&fmap_cache);
 		if (region_device_sz(&cache->rdev))
-			return rdev_chain(fmrd, &cache->rdev, 0,
-					  region_device_sz(&cache->rdev));
+			return rdev_chain_full(fmrd, &cache->rdev);
 	}
 
 	boot_device_init();