staging: iio: adc: ad7280a: use crc8.h API to build crc table

The custom build function ad7280_crc8_build_table is not needed. The
crc8_populate_msb function from linux/crc8.h will build the same crc
table.

Signed-off-by: Slawomir Stepien <sst@poczta.fm>
Acked-by: Lars-Peter Clausen <lars@metafoo.de>
Signed-off-by: Jonathan Cameron <Jonathan.Cameron@huawei.com>