board: gateworks: gw_ventana: fix mmc env dev

Fix the MMC env device for boards with eMMC by adding a
board_mmc_get_env_dev override to return the boot device as the
MMC env device.

Signed-off-by: Tim Harvey <tharvey@gateworks.com>
diff --git a/board/gateworks/gw_ventana/gw_ventana.c b/board/gateworks/gw_ventana/gw_ventana.c
index 99f52b9..68fcc0b 100644
--- a/board/gateworks/gw_ventana/gw_ventana.c
+++ b/board/gateworks/gw_ventana/gw_ventana.c
@@ -1281,3 +1281,8 @@
 	return 0;
 }
 #endif /* CONFIG_OF_BOARD_SETUP */
+
+int board_mmc_get_env_dev(int devno)
+{
+	return devno;
+}