gpio: mockup: Fix mode of debugfs files

This driver's debugfs files have had a read operation since commit
2a9e27408e12 ("gpio: mockup: rework debugfs interface"), but were
still being created with write-only mode bits.  Update them to
indicate that the files can also be read.

Signed-off-by: Zev Weiss <zev@bewilderbeest.net>
Fixes: 2a9e27408e12 ("gpio: mockup: rework debugfs interface")
Cc: stable@kernel.org # v5.1+
Signed-off-by: Bartosz Golaszewski <bartosz.golaszewski@linaro.org>