bgmac: propagate error codes in bgmac_probe()

bgmac_mii_register() and register_netdev() both return appropriate error
codes for the failures they would encounter, propagate this error code
instead of overriding the value with -ENOTSUPP which is not the correct
error code to return.

Signed-off-by: Florian Fainelli <f.fainelli@gmail.com>
Acked-by: Rafał Miłecki <zajec5@gmail.com>
Signed-off-by: David S. Miller <davem@davemloft.net>