wifi: wil6210: Do not use embedded netdev in wil6210_priv

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 wil6210_priv by converting it
into a pointer. Then use the leverage alloc_netdev_dummy() to allocate
the net_device object at wil_if_add(). The free of the device
occurs at wil_if_remove().

Link: https://lore.kernel.org/all/20240229225910.79e224cf@kernel.org/

 [1]
Signed-off-by: Breno Leitao <leitao@debian.org>
Signed-off-by: Kalle Valo <quic_kvalo@quicinc.com>
Link: https://msgid.link/20240503103304.339489-1-leitao@debian.org