iio:accel:bma180: Make LOW_PASS_FILTER_3DB_FREQUENCY shared_by_type

the property is not per-channel, but shared by type

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