hmi-controller: prepare for API simplification

The return values for most of the API functions will be removed and replaced by
asserts. So remove the return value checks. The end result will be the same:
These functions only fail for incorrect API usage, so basically the asserts are
moved from the hmi-controller into the shell.

Signed-off-by: Michael Olbrich <m.olbrich@pengutronix.de>