net: bgmac: allocate struct bgmac just once & don't copy it

So far were were allocating struct bgmac in 3 places: platform code,
bcma code and shared bgmac_enet_probe function. The reason for this was
bgmac_enet_probe:
1) Requiring early-filled struct bgmac
2) Calling alloc_etherdev on its own in order to use netdev_priv later

This solution got few drawbacks:
1) Was duplicating allocating code
2) Required copying early-filled struct
3) Resulted in platform/bcma code having access only to unused struct

Solve this situation by simply extracting some probe code into the new
bgmac_alloc function.

Signed-off-by: Rafał Miłecki <rafal@milecki.pl>
Reviewed-by: Florian Fainelli <f.fainelli@gmail.com>
Signed-off-by: David S. Miller <davem@davemloft.net>