ath5k: review RX descriptor functions

Reviewed RX descriptor functions against the HAL sources. Some minor changes:

  - check size before making changes to the descriptor

  - whitespace

  - add comments about 5210 timestamps. this needs to be adressed later!

  - FIFO overrun error only available on 5210

  - rs_phyerr should not be OR'ed

  - clear the whole ath5k_rx_status structure before using, instead of
    zeroing specific fields.

Signed-off-by: Bruno Randolf <br1@einfach.org>
Signed-off-by: John W. Linville <linville@tuxdriver.com>