net: zynq_gem: Correct SGMII enable bit setting

Correct the SGMII enable bit position to 27 instead
of 31.

Signed-off-by: Siva Durga Prasad Paladugu <sivadur@xilinx.com>
Signed-off-by: Michal Simek <michal.simek@xilinx.com>
Acked-by: Joe Hershberger <joe.hershberger@ni.com>