match_string() returns the array index of a matching string.
Use it instead of the open-coded implementation.

Signed-off-by: YueHaibing <yuehaibing@huawei.com>
Link: https://lkml.kernel.org/r/20191109034226.21044-1-yuehaibing@huawei.com


Signed-off-by: Stephen Boyd <sboyd@kernel.org>