iwlwifi: Fix handling of retry count of NO_ACK frames in iwl-{3945|agn}-rs

Make iwl-{3945|agn}-rs check for IEEE80211_TX_CTL_NO_ACK instead of
is_multicast_ether_addr when determining whether to use the lowest
rate, and set the retry count to 0 (total try count = 1) if
IEEE80211_TX_CTL_NO_ACK is set.

Signed-off-by: Gábor Stefanik <netrolller.3d@gmail.com>
Signed-off-by: John W. Linville <linville@tuxdriver.com>