staging:iio:adc:ad7152: Fix differential channel return value and increase delay.

In differential mode zero scale equals to 0x8000.
Increase readout delay by 1ms.

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>