ASoC: mxs-saif: check BUSY bit in hw_params() only if not mclk_in_use

If something else, typically a codec, has enabled mclk, the BUSY
bit may be set when hw_params() is called without this being an
error.  This check thus causes intermittent failures to configure
the sound device when used in such a manner.  Fix this by making
the test conditional on !saif->mclk_in_use.

Signed-off-by: Mans Rullgard <mans@mansr.com>
Signed-off-by: Mark Brown <broonie@kernel.org>