gpio: sysfs: fix gpio attribute-creation race

commit ebbeba120ab2ec6ac5f3afc1425ec6ff0b77ad6f upstream.

Fix attribute-creation race with userspace by using the default group
to create also the contingent gpio device attributes.

Fixes: d8f388d8dc8d ("gpio: sysfs interface")
Signed-off-by: Johan Hovold <johan@kernel.org>
Signed-off-by: Linus Walleij <linus.walleij@linaro.org>
[bwh: Backported to 3.2:
 - Adjust filenames, context
 - Use gpio_to_desc(), not gpiod_to_desc(), in gpio_is_visible()
 - gpio_is_visible() must return mode_t]
Signed-off-by: Ben Hutchings <ben@decadent.org.uk>