gpiolib: Update indentation in driver.rst for code excerpts

gpiolib: Update indentation in driver.rst for code excerpts

When TABs are being used to indent the code excerpts inside the bullet
lists some of the tools [vim in particular] fail to recognize it and
continue interpreting the special characters inside the quoted excerpt.

Update indentation in driver.rst for code excerpts to avoid
their special interpretation.

Signed-off-by: Andy Shevchenko <andriy.shevchenko@linux.intel.com>
Link: https://lore.kernel.org/r/20201007143817.76335-1-andriy.shevchenko@linux.intel.com
Signed-off-by: Jonathan Corbet <corbet@lwn.net>