qtnfmac: check for passed channel being NULL in MGMT_TX command

Parameters passed into .mgmt_tx callback may have a NULL channel in case
userspace wants to send a frame on current channel. Make sure this case
is handled, pass "freq==0" in case channel is not specififed to tell
wlan device to use current channel.

Signed-off-by: Igor Mitsyanko <igor.mitsyanko.os@quantenna.com>
Signed-off-by: Kalle Valo <kvalo@codeaurora.org>