i40e: Store the irq number in i40e_q_vector

Make it easy to figure out the IRQ number for a particular i40e_q_vector by
storing the assigned IRQ in the structure itself.

Signed-off-by: Joe Damato <jdamato@fastly.com>
Acked-by: Jesse Brandeburg <jesse.brandeburg@intel.com>
Acked-by: Sridhar Samudrala <sridhar.samudrala@intel.com>
Tested-by: Gurucharan <gurucharanx.g@intel.com> (A Contingent worker at Intel)
Signed-off-by: Tony Nguyen <anthony.l.nguyen@intel.com>