iio:accel:bmc150-accel: fix counting direction

In bmc150_accel_unregister_triggers() triggers should be unregistered in
reverse order of registration. Trigger registration starts with number 0,
counting up. In consequence, trigger number needs to be count down here.

Signed-off-by: Hartmut Knaack <knaack.h@gmx.de>
Reviewed-by: Octavian Purdila <octavian.purdila@intel.com>
Cc: <Stable@vger.kernel.org>
Signed-off-by: Jonathan Cameron <jic23@kernel.org>