drivers: net: xgene: Add helper function

This is a prepartion patch and adds xgene_enet_get_fpsel() helper
function to get buffer pool number.

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>