Fix hpet_(un)register_irq_handler() for emulation

Fix hpet_(un)register_irq_handler() for when CONFIG_HPET_EMULATE_RTC=n.  They
are provided macros that substitute value 0, but if they are called as
functions and the return value isn't checked, the following warnings appear:

	drivers/char/rtc.c: In function `rtc_init':
	drivers/char/rtc.c:1063: warning: statement with no effect
	drivers/char/rtc.c: In function `rtc_exit':
	drivers/char/rtc.c:1157: warning: statement with no effect

Signed-off-by: David Howells <dhowells@redhat.com>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>