nfp: protect from theoretical size overflows on HW descriptor ring

Use array_size() and store the size as full size_t to protect from
theoretical size overflow when handling HW descriptor rings.

Signed-off-by: Jakub Kicinski <jakub.kicinski@netronome.com>
Reviewed-by: Dirk van der Merwe <dirk.vandermerwe@netronome.com>
Signed-off-by: David S. Miller <davem@davemloft.net>