When st driver decodes protocol index received from raw data,
it does a value convert from "char" to "int". Because it's sign
extension from bit8 to bit32, the "int" value maybe minus, in
another word, the protocol index might be minus, but driver doesn't
filter such case and may continue access memory pointed by this
minus index.

This patch is to change the variable type of index from "int"
to "unsigned char", so that it avoids do such kind of type
conversion.

cc: liu chuansheng <chuansheng.liu@intel.com>
Signed-off-by: Nchanning <chao.bi@intel.com>
Signed-off-by: NGreg Kroah-Hartman <gregkh@linuxfoundation.org>