nfp: add support for .get_link_ksettings()

Read link speed from the BAR.  This provides very basic information
and works for both PFs and VFs.

Signed-off-by: Jakub Kicinski <jakub.kicinski@netronome.com>
Reviewed-by: Simon Horman <simon.horman@netronome.com>
Signed-off-by: David S. Miller <davem@davemloft.net>