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>

Documentation/sound/kernel-api/writing-an-alsa-driver.rst

@@ -3994,21 +3994,21 @@ Driver with A Single Source File
 
    Suppose you have a file xyz.c. Add the following two lines::
 
-  snd-xyz-objs := xyz.o
-  obj-$(CONFIG_SND_XYZ) += snd-xyz.o
+     snd-xyz-objs := xyz.o
+     obj-$(CONFIG_SND_XYZ) += snd-xyz.o
 
 2. Create the Kconfig entry
 
    Add the new entry of Kconfig for your xyz driver::
 
-  config SND_XYZ
-    tristate "Foobar XYZ"
-    depends on SND
-    select SND_PCM
-    help
-      Say Y here to include support for Foobar XYZ soundcard.
-      To compile this driver as a module, choose M here:
-      the module will be called snd-xyz.
+     config SND_XYZ
+       tristate "Foobar XYZ"
+       depends on SND
+       select SND_PCM
+       help
+         Say Y here to include support for Foobar XYZ soundcard.
+         To compile this driver as a module, choose M here:
+         the module will be called snd-xyz.
 
 The line ``select SND_PCM`` specifies that the driver xyz supports PCM.
 In addition to SND_PCM, the following components are supported for
@@ -4032,7 +4032,7 @@ located in the new subdirectory, sound/pci/xyz.
 1. Add a new directory (``sound/pci/xyz``) in ``sound/pci/Makefile``
    as below::
 
-  obj-$(CONFIG_SND) += sound/pci/xyz/
+     obj-$(CONFIG_SND) += sound/pci/xyz/
 
 
 2. Under the directory ``sound/pci/xyz``, create a Makefile::