rt2x00: Add support for retry rates

rt2800pci can handle different retry rates,
it will always step 1 rate down after a failed
transmission so creating the retry rate list
for mac80211 is quite simple.

Signed-off-by: Benoit PAPILLAULT <benoit.papillault@free.fr>
Signed-off-by: Ivo van Doorn <IvDoorn@gmail.com>
Signed-off-by: John W. Linville <linville@tuxdriver.com>