rt2x00: Don't count retries as failure

Link quality estimation became quite low for all rt2x00 drivers
because the number of retries it took to send the frame were
counted as failure.
This does not correspond to the legacy driver link quality calculation,
by not counting it we will send somewhat more optimistic values to
mac80211.

Signed-off-by: Ivo van Doorn <IvDoorn@gmail.com>
Signed-off-by: John W. Linville <linville@tuxdriver.com>