nfp: switch to dev_alloc_page()

Use the dev_alloc_page() networking helper to allocate pages
for RX packets.

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>