qtnfmac: lock access to h/w in tx path

Fix tx path regression. Lock should be held when queuing packets
to h/w fifos in order to properly handle configurations with
multiple enabled interfaces.

Signed-off-by: Sergey Matyukevich <sergey.matyukevich.os@quantenna.com>
Signed-off-by: Kalle Valo <kvalo@codeaurora.org>