iio:accel:bma180: Use modifier instead of index in channel specification

This driver was not complying with the ABI and the purpose of this patch
is to bring it inline so that userspace will correctly identify the channels.

Should use channel modifiers (X/Y/Z), not channel indices
timestamp channel has scan index 3, not 4

Signed-off-by: Peter Meerwald <pmeerw@pmeerw.net>
Cc: Kravchenko Oleksandr <x0199363@ti.com>
Cc: Stable@vger.kernel.org
Signed-off-by: Jonathan Cameron <jic23@kernel.org>