bgmac: use phy_mii_ioctl in ioctl handler

This works pretty much the same way, so avoid duplicating the code.

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