net: gianfar: correctly determine the number of queue groups

eTSEC of-nodes may have children which are not queue-group nodes. For
example new-style fixed-phy declarations. These where incorrectly
assumed to be additional queue-groups.

Change the search to filter out any nodes which are not queue-groups,
or have been disabled.

Signed-off-by: Tobias Waldekranz <tobias@waldekranz.com>
Signed-off-by: David S. Miller <davem@davemloft.net>