ice: change vfs.num_msix_per to vf->num_msix

vfs::num_msix_per should be only used as default value for
vf->num_msix. For other use cases vf->num_msix should be used, as VF can
have different MSI-X amount values.

Fix incorrect register index calculation. vfs::num_msix_per and
pf->sriov_base_vector shouldn't be used after implementation of changing
MSI-X amount on VFs. Instead vf->first_vector_idx should be used, as it
is storing value for first irq index.

Fixes: fe1c5ca2

 ("ice: implement num_msix field per VF")
Reviewed-by: Jacob Keller <jacob.e.keller@intel.com>
Signed-off-by: Michal Swiatkowski <michal.swiatkowski@linux.intel.com>
Reviewed-by: Simon Horman <horms@kernel.org>
Tested-by: Rafal Romanowski <rafal.romanowski@intel.com>
Signed-off-by: Tony Nguyen <anthony.l.nguyen@intel.com>