ionic: add a check for max SGs and SKB frags

Add a check of the queue's max_sg_elems against the maximum frags we
expect to see per SKB and take the smaller of the two as our max for
the queues' descriptor buffer allocations.

Signed-off-by: Shannon Nelson <shannon.nelson@amd.com>
Signed-off-by: David S. Miller <davem@davemloft.net>