ix: Fix accessing wrong register

Use IXGBE_GRC_BY_MAC(hw) macro instead of IXGBE_GRC as IXGBE_GRC's
address is different on Denverton platform.

This patch is part of change made in NetBSD kernel
by Masanobu Saitoh, NetBSD maintainer.

Differential Revision: https://reviews.freebsd.org/D19175
Approved by: erj