net: macb: Fixed reading MII_LPA register

If macb is gem and is gigabit capable, lpa value is not read from
the right register(MII_LPA) and is read from MII_STAT1000. This patch
fixes reading of the lpa value.

Signed-off-by: Radu Pirea <radu_nicolae.pirea@upb.ro>
Acked-by: Joe Hershberger <joe.hershberger@ni.com>