net: mdio: of: fix potential NULL pointer derefernce

of_find_mii_timestamper() returns NULL if no timestamper is found.
Therefore, guard the unregister_mii_timestamper() calls.

Fixes: 1dca22b1 ("net: mdio: of: Register discovered MII time stampers.")
Signed-off-by: Michael Walle <michael@walle.cc>
Acked-by: Richard Cochran <richardcochran@gmail.com>
Signed-off-by: Jakub Kicinski <kuba@kernel.org>