ath5k: add capability flag for phyerror counters

Chipsets since revision AR5213A (0x59) have hardware counters for PHY errors
which we can read directly from the registers. Older hardware has to use the RX
descriptor status to get a count of PHY errors. This will be used in several
places in the ANI implementation, so a flag is useful.

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