ASoC: msm8x16-wcd: prevent out of bounds access

Hardcoding the third argument in strnstr function
is resulting in out of bounds access. Set the third argument
to sizeof the character string passed as the first argument
to prevent out of bounds access.

CRs-Fixed: 832915
Change-Id: I61be88701340e271fd866e0e1801722dbe7d63ac
Signed-off-by: Aravind Kumar <akumark@codeaurora.org>