i40e: Fix descriptor count manipulation

Changing descriptor count via 'ethtool -G' is not persistent across resets.
When PF reset occurs, we roll back to the default value of vsi->num_desc,
which is used then in i40e_alloc_rings to set descriptor count. XDP does a
PF reset so when user has changed the descriptor count and load XDP
program, the default count will be back there.

To fix this:
  * introduce new VSI members - num_tx_desc and num_rx_desc in favour of
    num_desc
  * set them in i40e_set_ringparam to user's values
  * set them to default values in i40e_set_num_rings_in_vsi only when they
    don't have previous values

Signed-off-by: Maciej Fijalkowski <maciej.fijalkowski@intel.com>
Tested-by: Andrew Bowers <andrewx.bowers@intel.com>
Signed-off-by: Jeff Kirsher <jeffrey.t.kirsher@intel.com>