iio: chemical: vz89x: fix boolreturn.cocci warnings

drivers/iio/chemical/vz89x.c:119:9-10: WARNING: return of 0/1 in function 'vz89x_measurement_is_valid' with return type bool

 Return statements in functions returning bool should use
 true/false instead of 1/0.
Generated by: scripts/coccinelle/misc/boolreturn.cocci

CC: Matt Ranostay <mranostay@gmail.com>
Signed-off-by: Fengguang Wu <fengguang.wu@intel.com>
Signed-off-by: Jonathan Cameron <jic23@kernel.org>