drivers/rtc/rtc-cmos.c: propagate hpet_register_irq_handler() failure

If hpet_register_irq_handler() fails, cmos_do_probe() will incorrectly
return 0.

Reported-by: Julia Lawall <julia.lawall@lip6.fr>
Cc: John Stultz <john.stultz@linaro.org>
Cc: Grant Likely <grant.likely@linaro.org>
Cc: Rob Herring <robh+dt@kernel.org>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>