[][OpenWrt Dev][Fix mmc upgrading script]

[Description]
Fix a condition checking error in mmc upgrading script

[Release-log]
N/A

Change-Id: I9b8d8dceebe3df287cd369c6d4ae7d72c839ae6e
Reviewed-on: https://gerrit.mediatek.inc/c/openwrt/feeds/mtk_openwrt_feeds/+/6105790
diff --git a/target/linux/mediatek/base-files/lib/upgrade/mmc.sh b/target/linux/mediatek/base-files/lib/upgrade/mmc.sh
index fee9091..de7086b 100644
--- a/target/linux/mediatek/base-files/lib/upgrade/mmc.sh
+++ b/target/linux/mediatek/base-files/lib/upgrade/mmc.sh
@@ -146,7 +146,7 @@
 	[ -z "${kernel_dev}" -o $? -ne 0 ] && return 1
 
 	rootfs_dev=$(cat /sys/module/boot_param/parameters/upgrade_rootfs_part 2>/dev/null)
-	[ -z "${kernel_dev}" -o $? -ne 0 ] && return 1
+	[ -z "${rootfs_dev}" -o $? -ne 0 ] && return 1
 
 	rootfs_dev=$(block_dev_path "${rootfs_dev}")
 	[ -z "${rootfs_dev}" -o $? -ne 0 ] && return 1