bnxt: Report actual link speed, rather than hard-coded 100Gbs

We need to report the actual link speed, rather than a hard-coded
100Gbs for a variety of reasons, but most importantly, as reporting
100Gbs on a 10g link breaks lacp when this NIC is in an LACP bundle
with other vendors' NICs after e98ed8d99fd4 (lacp:
Simplify lacp_compose_key()), as the fake 100g puts this nic
into a different aggregation group.

Sponsored by: Netflix
Reviewed by: glebius
Tested by: glebius