drivers: net: xgene: Separate set_speed from mac_init

Since mac_init is too heavy to be called when the link changes,
moved the speed_set configuration to a new function and added
mac_ops->set_speed function pointer.  This function will be
called from adjust_link callback.

Added cases for 10/100 support for SGMII based 1G interface.

Signed-off-by: Iyappan Subramanian <isubramanian@apm.com>
Tested-by: Fushen Chen <fchen@apm.com>
Tested-by: Toan Le <toanle@apm.com>
Signed-off-by: David S. Miller <davem@davemloft.net>