usb: gadget: u_audio: Check return value from config_ep_by_speed()

In case config_ep_by_speed() returns error, endpoint descriptors
would not get populated. This results into NULL pointer dereference
when ep desc is accessed later. Fix this by bailing out set_alt if
config_ep_by_speed() API returns error.

Change-Id: I581d5e9fa7f3610e96e2e3705ab7e095a93a2a59
Signed-off-by: Hemant Kumar <hemantk@codeaurora.org>
Signed-off-by: Ajay Agarwal <ajaya@codeaurora.org>