rtc: cmos: Do not export alarm rtc_ops when we do not support alarms

When there is no IRQ configured for the RTC, the rtc-cmos code does not
support alarms, all alarm rtc_ops fail with -EIO / -EINVAL.

The rtc-core expects a rtc driver which does not support rtc alarms to
not have alarm ops at all. Otherwise the wakealarm sysfs attr will read
as empty rather then returning an error, making it impossible for
userspace to find out beforehand if alarms are supported.

A system without an IRQ for the RTC before this patch:
[root@localhost ~]# cat /sys/class/rtc/rtc0/wakealarm
[root@localhost ~]#

After this patch:
[root@localhost ~]# cat /sys/class/rtc/rtc0/wakealarm
cat: /sys/class/rtc/rtc0/wakealarm: No such file or directory
[root@localhost ~]#

This fixes gnome-session + systemd trying to use suspend-then-hibernate,
which causes systemd to abort the suspend when writing the RTC alarm fails.

BugLink: https://github.com/systemd/systemd/issues/9988


Signed-off-by: Hans de Goede <hdegoede@redhat.com>
Signed-off-by: Alexandre Belloni <alexandre.belloni@bootlin.com>