i40e: use minimal Rx and Tx ring buffers for kdump

Use the minimum of the number of descriptors thus we will allocate the
minimal ring buffers for kdump.

Signed-off-by: Coiby Xu <coxu@redhat.com>
Tested-by: Dave Switzer <david.switzer@intel.com>
Signed-off-by: Tony Nguyen <anthony.l.nguyen@intel.com>