net: mii_timestamper: check NULL in unregister_mii_timestamper()

Callers of unregister_mii_timestamper() currently check for NULL
value of mii_ts before calling it.

Place the NULL check inside unregister_mii_timestamper() and update
the callers accordingly.

Signed-off-by: Calvin Johnson <calvin.johnson@oss.nxp.com>
Reviewed-by: Andy Shevchenko <andy.shevchenko@gmail.com>
Suggested-by: Andy Shevchenko <andy.shevchenko@gmail.com>
Signed-off-by: Ioana Ciornei <ioana.ciornei@nxp.com>
Acked-by: Grant Likely <grant.likely@arm.com>
Signed-off-by: David S. Miller <davem@davemloft.net>