usb: gadget: f_fs: restore ffs_func_disable() functionality

The blamed commit made ffs_func_disable() always return -EINVAL as the
method calls ffs_func_set_alt() with the ``alt`` argument being
``(unsigned)-1``, which is always greater than MAX_ALT_SETTINGS.
Use the MAX_ALT_SETTINGS check just in the f->set_alt() code path,
f->disable() doesn't care about the ``alt`` parameter.

Make a surgical fix, but really the f->disable() code shall be pulled
out from ffs_func_set_alt(), the code will become clearer. A patch will
follow.

Note that ffs_func_disable() always returning -EINVAL made pixel6 crash
on USB disconnect.

Fixes: 2f550553 ("usb: gadget: f_fs: Add the missing get_alt callback")
Cc: stable <stable@kernel.org>
Reported-by: William McVicker <willmcvicker@google.com>
Signed-off-by: Tudor Ambarus <tudor.ambarus@linaro.org>
Link: https://lore.kernel.org/r/20240802140428.2000312-2-tudor.ambarus@linaro.orgSigned-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>

drivers/usb/gadget/function/f_fs.c

@@ -3731,10 +3731,10 @@ static int ffs_func_set_alt(struct usb_function *f,
 	struct ffs_data *ffs = func->ffs;
 	int ret = 0, intf;
 
-	if (alt > MAX_ALT_SETTINGS)
-		return -EINVAL;
-
 	if (alt != (unsigned)-1) {
+		if (alt > MAX_ALT_SETTINGS)
+			return -EINVAL;
+
 		intf = ffs_func_revmap_intf(func, interface);
 		if (intf < 0)
 			return intf;