iio:accel:stk8312: improve error handling

Improve error handling in the following ways:
  - set return value on error condition to an appropriate error code
  - return error code immediately in case of an error (slightly changes
    code structure)
  - pass up real error code
  - add missing error handling
  - return 0 when error have been caught already
  - put device back in active mode after error occurs

Signed-off-by: Hartmut Knaack <knaack.h@gmx.de>
Signed-off-by: Jonathan Cameron <jic23@kernel.org>