iio:common:ms_sensors:ms_sensors_i2c: rework CRC calculation helper

iio:common:ms_sensors:ms_sensors_i2c: rework CRC calculation helper

The CRC calculation always happens on 8 words which is why there is an
extra element in the prom array of struct ms_tp_dev. However, on ms5637 and
similar, only 7 words are readable.

Then, set MS_SENSORS_TP_PROM_WORDS_NB to 8 and stop passing a len parameter
to ms_sensors_tp_crc_valid as this simply hide the fact that it is
hardcoded.

Finally, use the newly introduced hw->prom_len to know how many words can be
read.

Signed-off-by: Alexandre Belloni <alexandre.belloni@bootlin.com>
Link: https://lore.kernel.org/r/20210109231148.1168104-5-alexandre.belloni@bootlin.com
Signed-off-by: Jonathan Cameron <Jonathan.Cameron@huawei.com>