wifi: qtnfmac: Use netdev dummy allocator helper

There is a new dummy netdev allocator, use it instead of
alloc_netdev()/init_dummy_netdev combination.

Using alloc_netdev() with init_dummy_netdev might cause some memory
corruption at the driver removal side.

Fixes: 61cdb09f

 ("wifi: qtnfmac: allocate dummy net_device dynamically")
Signed-off-by: Breno Leitao <leitao@debian.org>
Acked-by: Kalle Valo <kvalo@kernel.org>
Signed-off-by: David S. Miller <davem@davemloft.net>