net: stmmac: ptp: improve handling of aux_ts_lock lifetime

net: stmmac: ptp: improve handling of aux_ts_lock lifetime

The aux_ts_lock mutex is only required while the PTP clock has been
successfully registered.

stmmac_ptp_register() does not return any errors (as we don't wish to
prevent the netdev being opened if PTP fails), stmmac_ptp_unregister()
was coded to allow it to be called irrespective of whether PTP was
successfully registered or not.

Arrange for the aux_ts_lock mutex to be destroyed if the PTP clock
is not functional during stmmac_ptp_register(), and only destroy it
in stmmac_ptp_unregister() if we had a PTP clock registered.

Signed-off-by: Russell King (Oracle) <rmk+kernel@armlinux.org.uk>
Signed-off-by: Jakub Kicinski <kuba@kernel.org>