net: thunderx: Unembed netdev structure

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_devices from struct lmac by converting them
into pointers, and allocating them dynamically. Use the leverage
alloc_netdev() to allocate the net_device object at
bgx_lmac_enable().

The free of the device occurs at bgx_lmac_disable().

 Do not free_netdevice() if bgx_lmac_enable() fails after lmac->netdev
is allocated, since bgx_lmac_disable() is called if bgx_lmac_enable()
fails, and lmac->netdev will be freed there (similarly to lmac->dmacs).

Link: https://lore.kernel.org/all/20240229225910.79e224cf@kernel.org/ [1]
Signed-off-by: Breno Leitao <leitao@debian.org>
Link: https://patch.msgid.link/20240626173503.87636-1-leitao@debian.org
Signed-off-by: Jakub Kicinski <kuba@kernel.org>