ASoC: fsl: imx-pcm-fiq: Remove unused 'runtime' variable

Commit 68f9672b

 (ASoC: fsl: imx-pcm-fiq: remove bogus period delta calculation)
introduced the following build warning:

sound/soc/fsl/imx-pcm-fiq.c:53:26: warning: unused variable 'runtime' [-Wunused-variable]

Remove the unused 'runtime' variable.

Signed-off-by: Fabio Estevam <fabio.estevam@freescale.com>
Acked-by: Oskar Schirmer <oskar@scara.com>
Signed-off-by: Mark Brown <broonie@linaro.org>