usb: gadget: f_uac2: 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: Iaf9a0001973753988ff200edad3ed8587b6a4181
Signed-off-by: Hemant Kumar <hemantk@codeaurora.org>