test: firmware_class: use kstrndup() where appropriate

We're essentially just doing an open-coded kstrndup(). The only
differences are with what happens after the first '\0' character, but
request_firmware() doesn't care about that.

Suggested-by: Kees Cook <keescook@chromium.org>
Signed-off-by: Brian Norris <computersforpeace@gmail.com>
Signed-off-by: Shuah Khan <shuahkh@osg.samsung.com>