ASoC: rt5514: Fix the issue that the variable dereferenced before checking

The patch fixes the issue that variable dereferenced before checking
'rt5514_dsp->substream'. Move the assignment to after the variable
checking of 'rt5514_dsp->substream'.

Signed-off-by: Oder Chiou <oder_chiou@realtek.com>
Signed-off-by: Mark Brown <broonie@kernel.org>