ice: refactor interrupt moderation writes

Introduce several new helpers for writing ITR and GLINT_RATE
registers, and refactor the code calling them.  This resulted
in removal of several duplicate functions and rolled a bunch
of simple code back into the calling routines.

In particular this removes some code that was doing both
a store and a set in a helper function, which seems better
done as separate tasks in the caller (and generally takes
less lines of code even with a tiny bit of repetition).

Signed-off-by: Jesse Brandeburg <jesse.brandeburg@intel.com>
Tested-by: Tony Brelinski <tonyx.brelinski@intel.com>
Signed-off-by: Tony Nguyen <anthony.l.nguyen@intel.com>