igb: Fix reg pattern test in ethtool for i350 devices

This fixes the reg_pattern_test so that the test does not fail
on i350 parts.

Signed-off-by: Carolyn Wyborny <carolyn.wyborny@intel.com>
Tested-by: Jeff Pieper <jeffrey.e.pieper@intel.com>
Signed-off-by: Jeff Kirsher <jeffrey.t.kirsher@intel.com>