ASoC: rt5514: Fix uninitialized calibration value

With gcc-4.1.2, if CONFIG_SND_SOC_RT5514_SPI is not set:

    sound/soc/codecs/rt5514.c: In function ‘rt5514_dsp_voice_wake_up_put’:
    sound/soc/codecs/rt5514.c:363: warning: ‘buf[0]’ is used uninitialized in this function
    sound/soc/codecs/rt5514.c:363: warning: ‘buf[1]’ is used uninitialized in this function
    sound/soc/codecs/rt5514.c:363: warning: ‘buf[2]’ is used uninitialized in this function
    sound/soc/codecs/rt5514.c:363: warning: ‘buf[3]’ is used uninitialized in this function

Fix this by initializing the buffer with zeroes, to avoid random bits
being written to the calibration register later.

Fixes: fc9cab05 ("ASoC: rt5514: The DSP clock can be calibrated by the other clock source")
Signed-off-by: Geert Uytterhoeven <geert@linux-m68k.org>
Signed-off-by: Mark Brown <broonie@kernel.org>