e1000: Clean up ITR/EITR in preparation for AIM

Provide macros to derive the various needed values and make it a bit
more clear the differences between em and igb.

The igb default EITR was not landing at the right offset.

Respect the 'max_interrupt_rate' tunable.

Sponsored by: BBOX.io

(cherry picked from commit 9bf9164fc8aad1ca828c725413e06462aef1e4dd)