ptp: vmclock: Don't unregister misc device if it was not registered

vmclock_remove() tries to detect the successful registration of the misc
device based on the value of its minor value.
However that check is incorrect if the misc device registration was not
attempted in the first place.

Always initialize the minor number, so the check works properly.

Fixes: 20503272 ("ptp: Add support for the AMZNC10C 'vmclock' device")
Cc: stable@vger.kernel.org
Signed-off-by: Thomas Weißschuh <thomas.weissschuh@linutronix.de>
Acked-by: Richard Cochran <richardcochran@gmail.com>
Reviewed-by: David Woodhouse <dwmw@amazon.co.uk>
Signed-off-by: Paolo Abeni <pabeni@redhat.com>