enic: Replace hardcoded values for vnic descriptor by defines

Replace the hardcoded values used in the calculations for
vnic descriptors and rings with defines. Minor code cleanup.

Signed-off-by: Satish Kharat <satishkh@cisco.com>
Reviewed-by: Simon Horman <horms@kernel.org>
Signed-off-by: David S. Miller <davem@davemloft.net>