nl80211: Indicate driver-based offchannel TX on mgmt_tx_cancel_wait

Drivers that support frame transmission with mgmt_tx() may not support
driver-based offchannel TX. Use mgmt_tx_cancel_wait instead of mgmt_tx
when figuring out whether to indicate support for this with
NL80211_ATTR_OFFCHANNEL_TX_OK.

Signed-off-by: Jouni Malinen <jouni@qca.qualcomm.com>
Acked-by: Johannes Berg <johannes@sipsolutions.net>
Signed-off-by: John W. Linville <linville@tuxdriver.com>