usb: gadget: f_uac2: restructure some code in afunc_set_alt()

Restructure some code to make it easier to read.

While at it, return -ENOMEM instead of -EINVAL if
usb_ep_alloc_request() fails, and omit the logging in such cases
(the mm core will complain loud enough).

Signed-off-by: Daniel Mack <zonque@gmail.com>
Signed-off-by: Felipe Balbi <balbi@ti.com>