staging: iio: adc: ad7476 more list review feedback by Jonathan Cameron

Print in_scale in decimal format.
Kconfig list the names of the parts in full, to ease seraching.
Use st->spi in the call as it is only used the once.

Signed-off-by: Michael Hennerich <michael.hennerich@analog.com>
Acked-by: Jonathan Cameron <jic23@cam.ac.uk>
Signed-off-by: Greg Kroah-Hartman <gregkh@suse.de>