test_firmware: use kernel-doc struct notation

Add "struct" to the kernel-doc notation to prevent a warning:

lib/test_firmware.c:98: warning: cannot understand function prototype: 'struct test_config '

Signed-off-by: Randy Dunlap <rdunlap@infradead.org>
Cc: Luis Chamberlain <mcgrof@kernel.org>
Cc: Russ Weight <russell.h.weight@intel.com>
Cc: Kees Cook <keescook@chromium.org>
Cc: Greg Kroah-Hartman <gregkh@linuxfoundation.org>
Acked-by: Luis Chamberlain <mcgrof@kernel.org>
Link: https://lore.kernel.org/r/20230102211554.25629-1-rdunlap@infradead.org


Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>