i40evf: add missing rtnl_lock() around i40evf_set_interrupt_capability

The function calls netif_set_real_num_(tx|rx)_queues, both of which
should be done only under rntl lock. Unfortunately the
i40evf_init_task did not hold the rtnl_lock as necessary. This patch
adds the locking needed.

Change-ID: Ib72a21c3ce22b71a226b16f9bbe0f5f8cc3e849b
Signed-off-by: Jacob Keller <jacob.e.keller@intel.com>
Tested-by: Andrew Bowers <andrewx.bowers@intel.com>
Signed-off-by: Jeff Kirsher <jeffrey.t.kirsher@intel.com>