net: macb: Not all the GEM are gigabit capable

During the initialization of PHY the gigabit bit capable is set if the
controller is a GEM. However, for sama5d2 and sama5d4, the GEM is
configured to support only 10/100.

Improperly setting the GBE capability leads to an unresponsive MAC
controller. This patch fixes this behavior allowing using the gmac with
these SoCs.

Suggested-by: Nicolas Ferre <nicolas.ferre@atmel.com>
Signed-off-by: Gregory CLEMENT <gregory.clement@free-electrons.com>
Acked-by: Nicolas Ferre <nicolas.ferre@atmel.com>
Acked-by: Joe Hershberger <joe.hershberger@ni.com>
Reviewed-by: Andreas Bießmann <andreas.devel@googlemail.com>
[fixed minor checkpatch warning]
Signed-off-by: Andreas Bießmann <andreas.devel@googlemail.com>