iio: accel: sca3000: Use sign_extend32() instead of opencoding sign extension.

Whilst nice to get rid of this non obvious code, this also clears a
static checker warning:

drivers/iio/accel/sca3000.c:734 sca3000_read_raw()
warn: no-op. '((*val) << 19) >> 19'

Reported-by: Dan Carpenter <dan.carpenter@oracle.com>
Signed-off-by: Jonathan Cameron <Jonathan.Cameron@huawei.com>
Reviewed-by: Lars-Peter Clausen <lars@metafoo.de>
Link: https://lore.kernel.org/r/20211017160303.72441-1-jic23@kernel.org