nfp: abm: calculate PRIO map len and check mailbox size

In preparation for PRIO offload calculate how long the prio map
for FW will be and make sure the configuration can be performed
via the vNIC mailbox.

Signed-off-by: Jakub Kicinski <jakub.kicinski@netronome.com>
Reviewed-by: John Hurley <john.hurley@netronome.com>
Signed-off-by: David S. Miller <davem@davemloft.net>