staging:iio: gpio build dependency fixing

Numerous drivers either had pointless includes of gpio.h
or should have been dependent on GENERIC_GPIO and were not.
Conversion of ads1210 to use array registration triggered
build failures that highlighted all was not well.

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