ASoC: soc: prevent risk of buffer overflow

In case of large value for bufcnt_t or bufcnt,
cmd_size may overflow. Buffer size allocated by cmd_size might
be not as expected.
Possible buffer overflow could happen.

CRs-Fixed: 1084210
Change-Id: I9556f18dd6a9fdf3f76c133ae75c04ecce171f08
Signed-off-by: Xiaojun Sang <xsang@codeaurora.org>