usb: gadget: f_uac2: handle partial dma area wrap

With packet sizes other than 512, payloads in the packets may wrap
around the ALSA dma buffer partially, which leads to memory corruption
and audible clicks and pops in the audio stream at the moment, because
there is no boundary check before the memcpy().

In preparation to an implementation for smaller and dynamically sized
packets, we have to address such cases, and copy the payload in two
steps conditionally.

The 'src' and 'dst' approach doesn't work here anymore, as different
behavior is necessary in playback and capture cases. Thus, this patch
open-codes the routine now.

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