mediatek: mt7622: fix sysupgrade on MMC on BPi-R64

A previous attempt to simplify things went wrong and now sysupgrade
is broken on this device. Fix that.

Fixes: d640cba ("mediatek: mt7622: don't rely on existing image for sysupgrade")
Signed-off-by: Daniel Golle <daniel@makrotopia.org>

target/linux/mediatek/mt7622/base-files/lib/upgrade/platform.sh

@@ -8,6 +8,7 @@ platform_do_upgrade() {
 	bananapi,bpi-r64)
 		local rootdev="$(cmdline_get_var root)"
 		rootdev="${rootdev##*/}"
+		rootdev="${rootdev%p[0-9]*}"
 		case "$rootdev" in
 		mmc*)
 			CI_ROOTDEV="$rootdev"