rtc: simplify implementations of read_alarm

Since commit d68778b8 ("rtc: initialize output parameter for read
alarm to "uninitialized"") there is no need to explicitly set
unsupported members to -1. So drop the respective assignments from
drivers.

Signed-off-by: Uwe Kleine-König <uwe@kleine-koenig.org>
Signed-off-by: Alexandre Belloni <alexandre.belloni@free-electrons.com>