Mechanically convert to if_inc_counter().
Convert driver to if_get_counter method.

Submitted by:	rstone
Reviewed by:	Eric Joyner <ricera10 gmail.com>
Fix the ar724x PCI config space register read.

It was doing incorrect things with masks.  This was fixed in the
AR71xx codebase but it wasn't yet fixed in the AR724x code.

This ended up having config space reads return larger/incorrect values
in some situations.

Tested:

* AR7240

TODO:

* test ar7241, AR7242, and AR934x.