drivers: net: xgene: Protect indirect MAC access

This patch,

     - refactors mac read/write functions
     - adds lock to protect indirect mac access

Signed-off-by: Iyappan Subramanian <isubramanian@apm.com>
Signed-off-by: Quan Nguyen <qnguyen@apm.com>
Signed-off-by: David S. Miller <davem@davemloft.net>