i40e: Fix changing previously set num_queue_pairs for PFs

Currently, the i40e_vsi_setup_queue_map is basing the count of queues in
TCs on a VSI's alloc_queue_pairs member which is not changed throughout
any user's action (for example via ethtool's set_channels callback).

This implies that vsi->tc_config.tc_info[n].qcount value that is given
to the kernel via netdev_set_tc_queue() that notifies about the count of
queues per particular traffic class is constant even if user has changed
the total count of queues.

This in turn caused the kernel warning after setting the queue count to
the lower value than the initial one:

$ ethtool -l ens801f0
Channel parameters for ens801f0:
Pre-set maximums:
RX:             0
TX:             0
Other:          1
Combined:       64
Current hardware settings:
RX:             0
TX:             0
Other:          1
Combined:       64

$ ethtool -L ens801f0 combined 40

[dmesg]
Number of in use tx queues changed invalidating tc mappings. Priority
traffic classification disabled!

Reason was that vsi->alloc_queue_pairs stayed at 64 value which was used
to set the qcount on TC0 (by default only TC0 exists so all of the
existing queues are assigned to TC0). we update the offset/qcount via
netdev_set_tc_queue() back to the old value but then the
netif_set_real_num_tx_queues() is using the vsi->num_queue_pairs as a
value which got set to 40.

Fix it by using vsi->req_queue_pairs as a queue count that will be
distributed across TCs. Do it only for non-zero values, which implies
that user actually requested the new count of queues.

For VSIs other than main, stay with the vsi->alloc_queue_pairs as we
only allow manipulating the queue count on main VSI.

Fixes: bc6d33c8

 ("i40e: Fix the number of queues available to be mapped for use")
Co-developed-by: Maciej Fijalkowski <maciej.fijalkowski@intel.com>
Signed-off-by: Maciej Fijalkowski <maciej.fijalkowski@intel.com>
Co-developed-by: Przemyslaw Patynowski <przemyslawx.patynowski@intel.com>
Signed-off-by: Przemyslaw Patynowski <przemyslawx.patynowski@intel.com>
Signed-off-by: Eryk Rybak <eryk.roch.rybak@intel.com>
Tested-by: Tony Brelinski <tony.brelinski@intel.com>
Signed-off-by: Tony Nguyen <anthony.l.nguyen@intel.com>