staging:iio:adis16260: Simplify calibscale and caliboffset reading

All channels for this device have the same number of bits for calibscale and
caliboffset, there is no need to determine it dynamically based on the channel
type. Also there is no locking required since adis_read_reg_16() will take care
of proper locking on its own.

Signed-off-by: Lars-Peter Clausen <lars@metafoo.de>
Signed-off-by: Jonathan Cameron <jic23@kernel.org>