ixp4xx_eth: fix ptp_ixp46x build failure

When building with ixp4xx_eth and ptp_ixp46x as module, one is getting the
following error:

ERROR: "ixp46x_phc_index" [drivers/ptp/ptp_ixp46x.ko] undefined!

This has been introduced by commit 509a7c25.

Signed-off-by: Arnaud Patard <arnaud.patard@rtp-net.org>
Acked-by: Richard Cochran <richardcochran@gmail.com>
Signed-off-by: David S. Miller <davem@davemloft.net>