iio: sx9500: Add missing init in sx9500_buffer_pre{en,dis}able()

drivers/iio/proximity/sx9500.c: In function ‘sx9500_buffer_preenable’:
drivers/iio/proximity/sx9500.c:682: warning: ‘ret’ may be used uninitialized in this function
drivers/iio/proximity/sx9500.c: In function ‘sx9500_buffer_predisable’:
drivers/iio/proximity/sx9500.c:706: warning: ‘ret’ may be used uninitialized in this function

If active_scan_mask is empty, it will loop once more over all channels,
doing nothing.

Signed-off-by: Geert Uytterhoeven <geert@linux-m68k.org>
Reviewed-by: Vlad Dogaru <vlad.dogaru@intel.com>
Signed-off-by: Jonathan Cameron <jic23@kernel.org>