docs: gpio: driver.rst: don't mark literal blocks twice

Two literal blocks there are marked with both "::" and

	.. code-block:: c

This causes Sphinx (2.4.1) to do the wrong thing, causing
lots of warnings:

    Documentation/driver-api/gpio/driver.rst:425: WARNING: Unexpected indentation.
    Documentation/driver-api/gpio/driver.rst:423: WARNING: Inline emphasis start-string without end-string.
    Documentation/driver-api/gpio/driver.rst:427: WARNING: Block quote ends without a blank line; unexpected unindent.
    Documentation/driver-api/gpio/driver.rst:429: WARNING: Inline emphasis start-string without end-string.
    Documentation/driver-api/gpio/driver.rst:429: WARNING: Inline emphasis start-string without end-string.
    Documentation/driver-api/gpio/driver.rst:429: WARNING: Inline emphasis start-string without end-string.
    Documentation/driver-api/gpio/driver.rst:433: WARNING: Inline emphasis start-string without end-string.
    Documentation/driver-api/gpio/driver.rst:446: WARNING: Unexpected indentation.
    Documentation/driver-api/gpio/driver.rst:440: WARNING: Inline emphasis start-string without end-string.
    Documentation/driver-api/gpio/driver.rst:440: WARNING: Inline emphasis start-string without end-string.
    Documentation/driver-api/gpio/driver.rst:447: WARNING: Block quote ends without a blank line; unexpected unindent.
    Documentation/driver-api/gpio/driver.rst:449: WARNING: Definition list ends without a blank line; unexpected unindent.
    Documentation/driver-api/gpio/driver.rst:462: WARNING: Unexpected indentation.
    Documentation/driver-api/gpio/driver.rst:460: WARNING: Inline emphasis start-string without end-string.
    Documentation/driver-api/gpio/driver.rst:462: WARNING: Inline emphasis start-string without end-string.
    Documentation/driver-api/gpio/driver.rst:465: WARNING: Block quote ends without a blank line; unexpected unindent.
    Documentation/driver-api/gpio/driver.rst:467: WARNING: Inline emphasis start-string without end-string.
    Documentation/driver-api/gpio/driver.rst:467: WARNING: Inline emphasis start-string without end-string.
    Documentation/driver-api/gpio/driver.rst:467: WARNING: Inline emphasis start-string without end-string.
    Documentation/driver-api/gpio/driver.rst:471: WARNING: Inline emphasis start-string without end-string.
    Documentation/driver-api/gpio/driver.rst:478: WARNING: Inline emphasis start-string without end-string.

Signed-off-by: Mauro Carvalho Chehab <mchehab+huawei@kernel.org>
Signed-off-by: Bartosz Golaszewski <bgolaszewski@baylibre.com>