usb: gadget: f_uac1: check return code from config_ep_by_speed

Not checking config_ep_by_speed could lead to a kernel
NULL pointer dereference error in usb_ep_enable

Cc: Felipe Balbi <balbi@ti.com>
Signed-off-by: Michael Trimarchi <michael@amarulasolutions.com>
Signed-off-by: Felipe Balbi <balbi@ti.com>