iio:st_sensors: align on storagebits boundaries

Ensure triggered buffering memory accesses are properly aligned on per
channel storagebits boundaries.

Signed-off-by: Gregor Boirie <gregor.boirie@parrot.com>
Tested-by: Linus Walleij <linus.walleij@linaro.org>
Signed-off-by: Jonathan Cameron <jic23@kernel.org>