Staging: gdm724x: use match_string() helper

match_string() returns the index of an array for a matching string,
which can be used instead of open coded variant.

Cc: Greg Kroah-Hartman <gregkh@linuxfoundation.org>
Cc: Quytelda Kahja <quytelda@tamalin.org>
Cc: devel@driverdev.osuosl.org
Signed-off-by: Yisheng Xie <xieyisheng1@huawei.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>