Fix ig4 operation for certain machines (from DragonFly)

[grembo]

Summary

The ig4 driver was ported from DragonFly where the following
issues were found in the original driver:

• Some machine BIOSes use the I2C bus and leave it in a state that causes interrupts to not work properly due to a pending interrupt having been latched.

• Refactor the code a bit to clear pending interrupts when I2C is enabled. This fixes the primary problem.

• Also fix a possible race condition in the interrupt handler where the interrupt was being cleared after reading the status instead of before.

I didn't experience those issues myself, but I received one user report
recently that sounded like it could've been caused by this issue.

It would be great to get this fix into 11.0-RELEASE before the code freeze
in two weeks from now.

Links to the original patch (thanks to @pfg for bringing this to my attention):

http://freshbsd.org/commit/dfbsd/2010e26953b9dc96ac7986666ce25b0fbe38f1ca
http://gitweb.dragonflybsd.org/dragonfly.git/commit/2010e26953b9dc96ac7986666ce25b0fbe38f1ca

Test Plan

Compiled, installed and made sure everything still worked as expected.

[jhb]

Approved once you fix the comment nit.

sys/dev/ichiic/ig4_iic.c

563 ↗

(On Diff #16954)

The comment should be moved with the code and possibly updated due to its new location.

[grembo]

Fix comment as pointed out by @jhb