usb: musb: Fix NULL check on struct musb_request field

The 'request' variable is a pointer to the 'request' field of the
struct musb_request 'req' pointer. It only worked until now because
the 'request' field is the first one in the musb_request structure, but
as soon as that changes, the check will be invalid.

Fix it preventively by doing the NULL-check on the 'req' pointer
instead.

Suggested-by: Maarten ter Huurne <[e-mail]>
Acked-by: Tony Lindgren <[e-mail]>
Signed-off-by: Paul Cercueil <[e-mail]>
Link: https://lore.kernel.org/r/[e-mail]
Signed-off-by: Greg Kroah-Hartman <[e-mail]>

drivers/usb/musb/musb_gadget.c

···
 		return;
 	}
 
-	if (request) {
+	if (req) {
 
 		trace_musb_req_tx(req);