net: macb: enable GMAC IP without GE feature support

The User Register in GMAC IP is used to select interface type.
When with GE feature, it is used to select interface between
RGMII and GMII. If without GE feature, it is used to select
interface between MII and RMII.

Signed-off-by: Bo Shen <voice.shen@atmel.com>
Signed-off-by: Andreas Bießmann <andreas.devel@googlemail.com>