iio: st_sensors: Use get_unaligned_be24() and sign_extend32()

Use these functions instead of open-coding them.

Cc: Denis Ciocca <denis.ciocca@st.com>
Signed-off-by: Andy Shevchenko <andriy.shevchenko@linux.intel.com>
Signed-off-by: Jonathan Cameron <Jonathan.Cameron@huawei.com>