usb: gadget: function: u_ether: don't starve tx request queue

commit 6c83f77278f17a7679001027e9231291c20f0d8a upstream.

If we don't guarantee that we will always get an
interrupt at least when we're queueing our very last
request, we could fall into situation where we queue
every request with 'no_interrupt' set. This will
cause the link to get stuck.

The behavior above has been triggered with g_ether
and dwc3.

Reported-by: Ville Syrjälä <ville.syrjala@linux.intel.com>
Signed-off-by: Felipe Balbi <felipe.balbi@linux.intel.com>
Signed-off-by: Willy Tarreau <w@1wt.eu>
[@nathanchance: fixed merge conflicts]
[@MSF-Jarvis: Fixed merge conflicts]
Signed-off-by: Nathan Chancellor <natechancellor@gmail.com>

Change-Id: Idd21aab544cc937b43d9a471772696d3c5a2ef41