Prevent failure for non existing scale and offset attributes

Created by: spandruvada

build_channel_array() goes through each directory member in scan_elements folder and tries to get scale and offset of matching elements. But there will be no scale and offset for some elements like timestamp. For example: in_accel_x_en in_accel_y_en in_accel_z_en in_timestamp_en in_accel_x_index in_accel_y_index in_accel_z_index in_timestamp_index in_accel_x_type in_accel_y_type in_accel_z_type in_timestamp_type

When scale and offset not present, iioutils_get_param_float() will fail, this causes return from the build_channel_array().

It shouldn't return when error is caused when the attribute is not present. Like in this case in_time_stamp_scale. This is already fixed in iio_utils.c in linux source tree under tools/iio, so adding the same change here.