gpio: mockup: use simple_read_from_buffer() in debugfs read callback

Calling read() for a single byte read will return 2 currently. Use
simple_read_from_buffer() which correctly handles all sizes.

Fixes: 2a9e2740 ("gpio: mockup: rework debugfs interface")
Reviewed-by: Mukesh Ojha <mojha@codeaurora.org>
Signed-off-by: Bartosz Golaszewski <bgolaszewski@baylibre.com>