iflib: Add subinterface interrupt allocation function

[erj]

Summary

The ice(4) driver will add the ability to create extra interfaces
that hang off of the base interface; to do that the driver requires
a method for the subinterface to request hardware interrupt resources
from the base interface.

Fix a related function definition argument line size issue as well.

Signed-off-by: Eric Joyner <[e-mail]>