net: dsa: microchip: ptp: Fix error code in ksz_hwtstamp_set()

We want to return negative error codes here but the copy_to/from_user()
functions return the number of bytes remaining to be copied.

Fixes: c59e12a1

 ("net: dsa: microchip: ptp: Initial hardware time stamping support")
Signed-off-by: Dan Carpenter <error27@gmail.com>
Reviewed-by: Vladimir Oltean <olteanv@gmail.com>
Link: https://lore.kernel.org/r/Y8fJxSvbl7UNVHh/@kili
Signed-off-by: Jakub Kicinski <kuba@kernel.org>