gpio: mockup: fix coding style

I have missed two indentation issues in commit 64e7112e ("gpio:
mockup: add set_config to support pull up/down"). This commit fixes them.

Fixes: 64e7112e ("gpio: mockup: add set_config to support pull up/down")
Signed-off-by: Bartosz Golaszewski <bgolaszewski@baylibre.com>