net: stmmac: ptp: stmmac_enable(): move change of plat->flags into mutex

This is a preparation patch. The next patch will check if an external TS
is active and return with an error. So we have to move the change of the
plat->flags that tracks if external timestamping is enabled after that
check.

Prepare for this change and move the plat->flags change into the mutex
and the if (on).

Signed-off-by: Johannes Zink <j.zink@pengutronix.de>
Signed-off-by: Paolo Abeni <pabeni@redhat.com>