lantiq: properly release descriptors in etop driver detach

lantiq: properly release descriptors in etop driver detach

This patch fixes two issues in the driver detach:
* double free of the same descriptor (upstream bug, backported in 66177c081f1b05e4eb7f6c23d97954e8172c4336),
* releasing tx descriptor instead of rx (downstream bug).

The driver is compiled into the kernel that is why the error
is not visible in normal use.

Signed-off-by: Aleksander Jan Bajkowski <olek2@wp.pl>
Link: https://github.com/openwrt/openwrt/pull/15939
Signed-off-by: Hauke Mehrtens <hauke@hauke-m.de>