iio: sx9500: rework error handling of raw readings

Fix error handling so that we can power the chip down even if a raw read
fails.

Reported-by: Hartmut Knaack <knaack.h@gmx.de>
Signed-off-by: Vlad Dogaru <vlad.dogaru@intel.com>
Acked-by: Hartmut Knaack <knaack.h@gmx.de>
Signed-off-by: Jonathan Cameron <jic23@kernel.org>