ath5k: review and add comments for descriptors

I carefully reviewed desh.h against the HAL sources. Added comments and made
differences between 5210, 5211 and 5212 more clear by adding _521x to the
defines which are specific to that chipset. Renamed some defines. No functional
changes.

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