rtc: ds1307: fix alarm reading at probe time

With the actual code, read_alarm() always returns -EINVAL when called
during the RTC device registration. This prevents from retrieving an
already configured alarm in hardware.

This patch fixes the issue by moving the HAS_ALARM bit configuration
(if supported by the hardware) above the rtc_device_register() call.

Signed-off-by: Simon Guinot <simon.guinot@sequanux.org>
Signed-off-by: Alexandre Belloni <alexandre.belloni@free-electrons.com>