env: sf: Use ENV_VALID enum names instead of literals

The function env_sf_init_addr() assigns number literals (1) instead of
ENV_VALID to gd->env_valid. Fix this.
Signed-off-by: NMarek Behún <marek.behun@nic.cz>
Reviewed-by: NSimon Glass <sjg@chromium.org>

env/sf.c

@@ -339,10 +339,10 @@ static int env_sf_init_addr(void)
 
 	if (crc32(0, env_ptr->data, ENV_SIZE) == env_ptr->crc) {
 		gd->env_addr = (ulong)&(env_ptr->data);
-		gd->env_valid = 1;
+		gd->env_valid = ENV_VALID;
 	} else {
 		gd->env_addr = (ulong)&default_environment[0];
-		gd->env_valid = 1;
+		gd->env_valid = ENV_VALID;
 	}
 
 	return 0;