ALSA: docs: Fix code block indentation in ALSA driver example

Sphinx reports htmldocs warnings:

Documentation/sound/kernel-api/writing-an-alsa-driver.rst:3997: WARNING: Literal block expected; none found.
Documentation/sound/kernel-api/writing-an-alsa-driver.rst:4004: WARNING: Literal block expected; none found.
Documentation/sound/kernel-api/writing-an-alsa-driver.rst:4009: WARNING: Unexpected indentation.
Documentation/sound/kernel-api/writing-an-alsa-driver.rst:4035: WARNING: Literal block expected; none found.

These are due to indentation of example driver snippets which is outside
the code block scope.

Fix these by indenting code blocks in question to the scope.

Fixes: 4d421eeb ("ALSA: docs: writing-an-alsa-driver.rst: polishing")
Reported-by: kernel test robot <lkp@intel.com>
Closes: https://lore.kernel.org/linux-doc/202305021822.4U6XOvGf-lkp@intel.com/Signed-off-by: Bagas Sanjaya <bagasdotme@gmail.com>
Link: https://lore.kernel.org/r/20230503035416.62722-1-bagasdotme@gmail.comSigned-off-by: Takashi Iwai <tiwai@suse.de>