[][kernel][mt7988][eth][Correct CPU next free and last free index of DMAD for TX ring debug command]

[Description]
Fix CPU next free and last free index for TX ring debug command.

Without this patch, users are unable to retrieve correct CPU next free
and last free index of DMAD.

[Release-log]
N/A


Change-Id: I3219b2b705acad3372b630c3f787dabfe492d1ad
Reviewed-on: https://gerrit.mediatek.inc/c/openwrt/feeds/mtk_openwrt_feeds/+/8957080
1 file changed