bgmac: add support for new BGMAC_CMDCFG_SR position on core rev >= 4

The BGMAC_CMDCFG_SR register is at a different position on core rev >= 4
We do not know where this register is on a rev 5 or higher core, I have
newer seen such a core.

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