rt2800: correctly report MCS TX parameters

We should only set IEEE80211_HT_MCS_TX_RX_DIF when TX and RX MCS sets
are not equal, i.e. when number of tx streams is different than
number of RX streams.

Signed-off-by: Stanislaw Gruszka <sgruszka@redhat.com>
Signed-off-by: Kalle Valo <kvalo@codeaurora.org>