ASoC: Add a sanity check before using dai driver name

The dai driver's name is allowed to be NULL. So add a sanity check for
that.

Signed-off-by: Jeffy Chen <jeffy.chen@rock-chips.com>
Reported-by: Donglin Peng <dolinux.peng@gmail.com>
Signed-off-by: Mark Brown <broonie@kernel.org>