staging:iio:adc:ad7606 make gpio request failures more consistent

To my mind, if a gpio is specified in the board file, yet fails
to be successfully requested, that is an error condidtion and
the driver should not muddle on regardless.

This does mean unwinding the gpios on error. Also the free_gpios
function is reordered so that it is consistent with the request one
(reverse order obviously).

This patch is the category of not technically fixing anything, just
making the driver be more in line with what a reviewer will expect.

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