net: macb: Add support for 1000-baseX

Macb can be used with Xilinx PCS/PMA PHY in fpga which is a 1000-baseX
phy(lpa 0x41e0). This patch adds checks for LPA_1000XFULL and
LPA_1000XHALF bits.

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