iio:st_pressure: align storagebits on power of 2

Sampled pressure data are 24 bits long and should be stored in a 32 bits
word.

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>