MLK-11915-12 ASoC: fsl_asrc_m2m: fix null check issue for variable m2m and pair

MLK-11915-12 ASoC: fsl_asrc_m2m: fix null check issue for variable m2m and pair

After allocating memory for m2m, we should null check for m2m instead of pair.

In fsl_asrc_close(), null-checking pair suggests that it may be null, but it
has already been dereferenced before the null check. pair will be alloceted
in fsl_asrc_open(), pair is null means that open dev file failed, and
close should not be called in user space. So remove null check for pair.

buf_len should not greater than ASRC_DMA_BUFFER_SIZE, otherwith dma buffer will
be overrun.

Reported by Coverity.

Signed-off-by: Zidan Wang <zidan.wang@freescale.com>
(cherry picked from commit b0dc15375b12b6c1bf46b9071b92267b827d8ce0)