enic: Use receive queue buffer blocks of 32/64 entries

Change the receive queue buffer allocations into blocks of 32 entries when
ring size is less than 64, otherwise use 64 entries per block.

Signed-off-by: Scott Feldman <scofeldm@cisco.com>
Signed-off-by: Vasanthy Kolluri <vkolluri@cisco.com>
Signed-off-by: Roopa Prabhu <roprabhu@cisco.com>
Signed-off-by: David S. Miller <davem@davemloft.net>