IB/core: Add generic function to extract IB speed from netdev

Logic of retrieving netdev speed from net_device and translating it to
IB speed is implemented in rxe, in usnic and in bnxt drivers.

Define new function which merges all.

Signed-off-by: Yuval Shaia <yuval.shaia@oracle.com>
Reviewed-by: Christian Benvenuti <benve@cisco.com>
Reviewed-by: Selvin Xavier <selvin.xavier@broadcom.com>
Reviewed-by: Moni Shoua <monis@mellanox.com>
Signed-off-by: Doug Ledford <dledford@redhat.com>