igb: fix NULL derefs due to skipped SR-IOV enabling

The combined effect of commits 6423fc34 ("igb: do not re-init SR-IOV
during probe") and ceee3450 ("igb: make sure SR-IOV init uses the
right number of queues") causes VFs no longer getting set up, leading
to NULL pointer dereferences due to the adapter's ->vf_data being NULL
while ->vfs_allocated_count is non-zero. The first commit not only
neglected the side effect of igb_sriov_reinit() that the second commit
tried to account for, but also that of setting IGB_FLAG_HAS_MSIX,
without which igb_enable_sriov() is effectively a no-op. Calling
igb_{,re}set_interrupt_capability() as done here seems to address this,
but I'm not sure whether this is better than sinply reverting the other
two commits.

Signed-off-by: Jan Beulich <jbeulich@suse.com>
Tested-by: Aaron Brown <aaron.f.brown@intel.com>
Signed-off-by: Jeff Kirsher <jeffrey.t.kirsher@intel.com>