clocksource/drivers/tango_xtal: Convert init function to return error

The init functions do not return any error. They behave as the following:

  - panic, thus leading to a kernel crash while another timer may work and
       make the system boot up correctly

  or

  - print an error and leave 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 returns an error code from the init function.

Signed-off-by: Daniel Lezcano <daniel.lezcano@linaro.org>
Acked-by: Marc Gonzalez <marc_gonzalez@sigmadesigns.com>