i40e: Add define for interrupt name string len

This patch creates a define for interrupt name string configuration that
is large enough to contain full bus/slot info, rather than just netdev->name.

Change-ID: Iaac0d23dfb8526defeed69d91cea85ed4a50ddb2
Signed-off-by: Carolyn Wyborny <carolyn.wyborny@intel.com>
Signed-off-by: Jeff Kirsher <jeffrey.t.kirsher@intel.com>