ionic: Change default number of descriptors for Tx and Rx

Cut down the number of default Tx and Rx descriptors to save
initial memory requirements.

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