carl9170: fix frame drop and WARN due to minstrel_ht change

carl9170: fix frame drop and WARN due to minstrel_ht change

With "mac80211/minstrel_ht: add support for using CCK rates"
minstrel_ht selects legacy CCK rates as viable rates for
outgoing frames which might be sent as part of an A-MPDU
[IEEE80211_TX_CTL_AMPDU is set].

This behavior triggered the following WARN_ON in the driver:
> WARNING: at carl9170/tx.c:995 carl9170_op_tx+0x1dd/0x6fd
The driver assumed that the rate control algorithm made a
mistake and dropped the frame.

This patch removes the noisy warning altogether and allows
said A-MPDU frames with CCK sample and/or fallback rates to
be transmitted seamlessly.

Signed-off-by: Christian Lamparter <chunkeey@googlemail.com>
Signed-off-by: John W. Linville <linville@tuxdriver.com>