clockevents: Repair kernel-doc for clockevent_delta2ns()

Since the introduction of clockevents, i.e., commit d316c57f
("clockevents: add core functionality"), there has been a mismatch between
the function and the kernel-doc comment for clockevent_delta2ns().

Hence, ./scripts/kernel-doc -none kernel/time/clockevents.c warns about it.

Adjust the kernel-doc comment for clockevent_delta2ns() for make W=1
happiness.

Signed-off-by: Lukas Bulwahn <lukas.bulwahn@gmail.com>
Signed-off-by: Thomas Gleixner <tglx@linutronix.de>
Link: https://lore.kernel.org/r/20221102091048.15068-1-lukas.bulwahn@gmail.com