staging: iio: Fix scan_element naming issue

The addition of a number to the scan_element names caused
an issue in drivers that used either #define or an enum
to provide the number. Before this fix names like
ADIS16350_ACCEL_X_accel_x_en occur rather than 5_accel_x_en.

Signed-off-by: Jonathan Cameron <jic23@cam.ac.uk>
Signed-off-by: Greg Kroah-Hartman <gregkh@suse.de>