rt2x00: separte clearing entry from rt2x00lib_txdone

This makes rt2x00lib_txdone a bit simpler and will allow to reuse
code in different variant of txdone which I'm preparing.

Signed-off-by: Stanislaw Gruszka <sgruszka@redhat.com>
Signed-off-by: Kalle Valo <kvalo@codeaurora.org>