usb: gadget: f_subset: switch over to PTR_RET

this patch fixes the following Coccinelle warning:

drivers/usb/gadget/f_subset.c:279:8-14: WARNING: \
	PTR_RET can be used

Signed-off-by: Felipe Balbi <balbi@ti.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>