Identify the clocksources used by ptp_kvm by setting the clocksource ID
enum constants. This avoids dereferencing struct clocksource. Once the
system_counterval_t.cs member will be removed, this will also avoid the
need to obtain clocksource pointers from kvm_arch_ptp_get_crosststamp().

The clocksource IDs are associated to timestamps requested from the KVM
hypervisor, so the proper clocksource ID is known at the ptp_kvm request
site.

While at it, also make the ptp_kvm_get_time_fn() 'ret' variable type int as
that's what the function return value is.
Signed-off-by: Peter Hilber <peter.hilber@opensynergy.com>
Signed-off-by: Thomas Gleixner <tglx@linutronix.de>
Link: https://lore.kernel.org/r/20240201010453.2212371-6-peter.hilber@opensynergy.com