clocksource/drivers/time-efm32: Convert init function to return error

The init functions do not return any error and let the caller unaware of
the state of the system.

Change that by converting the init functions to return an error conforming
to the CLOCKSOURCE_OF_RET prototype.

Proper error handling (rollback, errno value) will be changed later case
by case, thus this change just return back an error or success in the init
function.

Signed-off-by: Daniel Lezcano <daniel.lezcano@linaro.org>
Acked-by: Uwe Kleine-König <u.kleine-koenig@pengutronix.de>