nfp: add helpers for FEC support

Implement helpers to determine and modify FEC modes via the NSP.
The NSP advertises FEC capabilities on a per port basis and provides
support for:
* Auto mode selection
* Reed Solomon
* BaseR
* None/Off

Signed-off-by: Dirk van der Merwe <dirk.vandermerwe@netronome.com>
Signed-off-by: Simon Horman <simon.horman@netronome.com>
Signed-off-by: David S. Miller <davem@davemloft.net>