nfp: flower: use host context count provided by firmware

Read the host context count symbols provided by firmware and use
it to determine the number of allocated stats ids. Previously it
won't be possible to offload more than 2^17 filter even if FW was
able to do so.

Signed-off-by: Pieter Jansen van Vuuren <pieter.jansenvanvuuren@netronome.com>
Reviewed-by: Jakub Kicinski <jakub.kicinski@netronome.com>
Signed-off-by: David S. Miller <davem@davemloft.net>