igb: do not use phy ops in ethtool test cleanup for non-copper parts

Currently the igb driver is experiencing a panic due to a null function
pointer being used during the cleanup of the ethtool looback test on
fiber/serdes parts.  This patch prevents that and adds a check prior to
calling any phy function.

Signed-off-by: Alexander Duyck <alexander.h.duyck@intel.com>
Signed-off-by: Jeff Kirsher <jeffrey.t.kirsher@intel.com>
Signed-off-by: David S. Miller <davem@davemloft.net>