bfin_mac: only advertise hardware time stamped when enabled.

The hardware time stamping code is a compile time option for the blackfin.
When it is not enabled, the driver should fall back to the standard
ethtool reply to the get_ts_info query.

Compile tested only.

Signed-off-by: Richard Cochran <richardcochran@gmail.com>
Tested-by: Bob Liu <lliubbo@gmail.com>
Signed-off-by: David S. Miller <davem@davemloft.net>