ASoC: soc-generic-dmaengine-pcm: separate max_buffer_size assignment

The config->pcm_hardware may be NULL when config->prealloc_buffer_size
is not zero, so it is better to move max_buffer_size assignment under
a separate condition.

Signed-off-by: Shengjiu Wang <shengjiu.wang@nxp.com>
Link: https://lore.kernel.org/r/1642646642-15908-1-git-send-email-shengjiu.wang@nxp.com


Signed-off-by: Mark Brown <broonie@kernel.org>