iio: imu: kmx61: Don't ignore kmx61_set_power_state errors

..except while in an error handler, where there is nothing
to be done anyway.

Signed-off-by: Daniel Baluta <daniel.baluta@intel.com>
Reviewed-by: Hartmut Knaack <knaack.h@gmx.de>
Signed-off-by: Jonathan Cameron <jic23@kernel.org>