[ALSA] fix typos in writing-an-alsa-driver

        You need to pass the <function>snd_dma_pci_data(pci)</function>,

        where pci is the struct <structname>pci_dev</structname> pointer

        of the chip as well.

        The <type>snd_sg_buf_t</type> instance is created as

        The <type>struct snd_sg_buf</type> instance is created as

        substream->dma_private. You can cast

        the pointer like: 

        <informalexample>

          <programlisting>

<![CDATA[

  struct snd_sg_buf *sgbuf = (struct snd_sg_buf_t*)substream->dma_private;

  struct snd_sg_buf *sgbuf = (struct snd_sg_buf *)substream->dma_private;

]]>

          </programlisting>

        </informalexample>