net: cavium: fix driver name

The driver name gets exposed in sysfs under /sys/bus/pci/drivers
so it should look like other devices. Change it to be common
format (instead of "Cavium PTP").

This is a trivial fix that was observed by accident because
Debian kernels were building this driver into kernel (bug).

Signed-off-by: Stephen Hemminger <stephen@networkplumber.org>
Signed-off-by: David S. Miller <davem@davemloft.net>