iio: imu: kmx61: Enhance error handling

This fixes parts of kmx61 error handling to make code easier to read and to be
more consistent with IIO coding conventions:
	* prefer as single point for error handling instead of duplicating code
	for each function
	* directly return a value from a case branch instead of breaking
	* fix error message for writing REG_CTRL1

Also, add separate error paths for kmx61_trigger_setup/iio_triggered_buffer_setup
calls.

Signed-off-by: Daniel Baluta <daniel.baluta@intel.com>
Signed-off-by: Jonathan Cameron <jic23@kernel.org>