net: Don't default Cavium PTP driver to 'y'

8c56df37 ("net: add support for Cavium PTP coprocessor") added the
Cavium PTP coprocessor driver and enabled it by default.  Remove the
"default y" because the driver only applies to Cavium ThunderX processors.

Fixes: 8c56df37 ("net: add support for Cavium PTP coprocessor")
Signed-off-by: Bjorn Helgaas <bhelgaas@google.com>
Signed-off-by: David S. Miller <davem@davemloft.net>