ath5k: remove pointless rx error overlay struct

ath5k_hw_rx_error was only used once, where we could easily just use
ath5k_hw_rx_status as well, so remove it.

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