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.