drivers: net: phy: xgene: Add lock to protect mac access

This patch,

- refactors mac access routine
- adds lock to protect mac indirect access

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