iio-sensors: Fix build warnings

iio-sensors: Fix build warnings

This patch fixed the following build warnings:

	enumeration.c: In function ‘setup_trigger_names’:
	enumeration.c:687:6: warning: unused variable ‘len’ [-Wunused-variable]

	control.c: In function ‘start_acquisition_thread’:
	control.c:560:6: warning: variable ‘ret’ set but not used [-Wunused-but-set-variable]

	control.c: In function ‘reapply_sampling_rates’:
	control.c:846:18: warning: unused variable ‘user’ [-Wunused-variable]

	control.c: In function ‘sensor_poll’:
	control.c:1348:6: warning: unused variable ‘uncal_start’ [-Wunused-variable]

	gyro-calibration.c: In function ‘gyro_cal_init’:
	gyro-calibration.c:33:6: warning: variable ‘ret’ set but not used [-Wunused-but-set-variable]

	filtering.c: In function ‘denoise_median’:
	filtering.c:111:8: warning: unused variable ‘scale’ [-Wunused-variable]
	filtering.c:110:14: warning: unused variable ‘z’ [-Wunused-variable]
	filtering.c:110:11: warning: unused variable ‘y’ [-Wunused-variable]
	filtering.c:110:8: warning: unused variable ‘x’ [-Wunused-variable]

	filtering.c: In function ‘denoise_average’:
	filtering.c:144:6: warning: unused variable ‘i’ [-Wunused-variable]

	sens.c: In function ‘main’:
	sens.c:507:19: warning: unused variable ‘tmp’ [-Wunused-variable]

Change-Id: I856b6ebe7869ddb8e9e38907483ec2750c9ae48e
Signed-off-by: Daniel Baluta <daniel.baluta@intel.com>