tcm_usb_gadget: Fix enabled attribute failure

Fix up tcm_usbg_tpg_store_enable() return value to propagate
usbg_attach() failure up to user-space if no HDC is found.

Reported-by: Andrzej Pietrasiewicz <andrzej.p@samsung.com>
Signed-off-by: Andrzej Pietrasiewicz <andrzej.p@samsung.com>
Acked-by: Sebastian Andrzej Siewior <bigeasy@linutronix.de>
Signed-off-by: Nicholas Bellinger <nab@linux-iscsi.org>