staging:iio:adc:ad7152: Miscellaneous fixes and touch-up

Remove unused define.
Introduce cached SETUP variables.
Wait until calibration finished. (Device returns to idle state)
IIO_CHAN_INFO_CALIBSCALE_SEPARATE use proper scales. (range 1.0 to 1.99999)
i2c_smbus word transactions expect low byte first, therefore swap bytes.
CAPDIFF is bit in SETUP not CFG.

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