Only unmap the RX mbuf DMA map if there's a buffer here.

The normal RX path (ath_rx_pkt()) will sync and unmap the
buffer before passing it up the stack.  We only need to do this
if we're flushing the FIFO during reset/shutdown.
Use ATH_MAX_SCATTER rather than ATH_TXDESC.

ATH_MAX_SCATTER is used to size the ath_buf DMA segment array.
We thus should use it when checking sizes of things.