nfp: devlink port split support for 1x100G CXP NIC

This commit makes it possible to use devlink to split the 100G CXP
Netronome into two 40G interfaces. Currently when you ask for 2
interfaces, the math in src/nfp_devlink.c:nfp_devlink_port_split
calculates that you want 5 lanes per port because for some reason
eth_port.port_lanes=10 (shouldn't this be 12 for CXP?). What we really
want when asking for 2 breakout interfaces is 4 lanes per port. This
commit makes that happen by calculating based on 8 lanes if 10 are
present.

Signed-off-by: Ryan C Goodfellow <rgoodfel@isi.edu>
Reviewed-by: Jakub Kicinski <jakub.kicinski@netronome.com>
Reviewed-by: Greg Weeks <greg.weeks@netronome.com>
Signed-off-by: David S. Miller <davem@davemloft.net>