ASoC: kirkwood-i2s: fix a compilation warning

In the function kirkwood_set_rate, when the rate cannot be satisfied
by the internal nor by an external clock, the clock source in undefined:

 warning: ‘clks_ctrl’ may be used uninitialized in this function

The ALSA subsystem should never gives such a rate because:
- the rates with the internal clock are limited to 44.1, 48 and 96 kHz
  as specified by the kirkwood_i2s_dai structure,
- the other rates are proposed in the structure kirkwood_i2s_dai_extclk
  only when the external clock is present.

In case of programming error (bad rate for internal clock and no
external clock), the function will simply cause a backtrace.

Signed-off-by: Jean-Francois Moine <moinejf@free.fr>
Signed-off-by: Mark Brown <broonie@linaro.org>