vdso/gettimeofday: Return bool from clock_gettime() helpers

The internal helpers are effectively using boolean results,
while pretending to use error numbers.

Switch the return type to bool for more clarity.

Signed-off-by: Thomas Weißschuh <thomas.weissschuh@linutronix.de>
Signed-off-by: Thomas Gleixner <tglx@linutronix.de>
Link: https://lore.kernel.org/all/20250701-vdso-auxclock-v1-6-df7d9f87b9b8@linutronix.de