wifi: ath10k: allocate dummy net_device dynamically

Embedding net_device into structures prohibits the usage of flexible
arrays in the net_device structure. For more details, see the discussion
at [1].

Un-embed the net_device from struct ath10k by converting it
into a pointer. Then use the leverage alloc_netdev() to allocate the
net_device object at ath10k_core_create(). The free of the device occurs
at ath10k_core_destroy().

[1] https://lore.kernel.org/all/20240229225910.79e224cf@kernel.org/

Signed-off-by: Breno Leitao <leitao@debian.org>
Acked-by: Kalle Valo <kvalo@kernel.org>
Signed-off-by: David S. Miller <davem@davemloft.net>