rtwn: fix Tx ring cleanup.

Do not try to clear stale Tx descriptor entries when there are some
running vaps; just free node references - rtwn_pci_tx_done() will free
mbufs without creating holes in the Tx descriptor space.
Also, reset only 2 first entries in the beacon ring - other will not be
used anyway.

Tested with RTL8188CE, STA + STA mode.