staging: iio: adc: remove explicit comparison to NULL

Use !condition instead off condition == NULL when comparing with NULL

Signed-off-by: Ioana Ciornei <ciorneiioana@gmail.com>
Reviewed-by: Daniel Baluta <daniel.baluta@intel.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>