[][kernel][mt7988][emmc][platform.sh: Align upgrade script with mt7986's]

[Description]
Change and align upgrade script platform.sh with mt7986's

[Release-log]
N/A

Change-Id: I32711de8e055c6149a887bf8231371e77bd2cc07
Reviewed-on: https://gerrit.mediatek.inc/c/openwrt/feeds/mtk_openwrt_feeds/+/7907321
2 files changed