wifi: rtw89: Un-embed dummy device

wifi: rtw89: Un-embed dummy device

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 the private struct by converting it
into a pointer. Then use the leverage the new alloc_netdev_dummy()
helper to allocate and initialize dummy devices.

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

Tested with RTL8852BE, RTL8852C and RTL8922AE.

Signed-off-by: Breno Leitao <leitao@debian.org>
Acked-by: Ping-Ke Shih <pkshih@realtek.com>
Tested-by: Ping-Ke Shih <pkshih@realtek.com>
Signed-off-by: Ping-Ke Shih <pkshih@realtek.com>
Link: https://msgid.link/20240424182351.3936556-1-leitao@debian.org