posix-cpu-timers: Replace old expiry retrieval in posix_cpu_timer_set()

Reuse the split out __posix_cpu_timer_get() function which does already the
right thing.

No functional change.

Signed-off-by: Thomas Gleixner <tglx@linutronix.de>
Signed-off-by: Frederic Weisbecker <frederic@kernel.org>
Reviewed-by: Anna-Maria Behnsen <anna-maria@linutronix.de>
Acked-by: Peter Zijlstra (Intel) <peterz@infradead.org>