dwc3: gadget: Check available trbs before processing req

if number of trbs not sufficient while processing request with
scatter/gather buffers some buffers may be dropped. To avoid
dropping packets check for available trbs before processing
number of trbs.

Change-Id: Ia85e2b57d20117d2319923b59c78b69c26385e42
Signed-off-by: Vamsi Krishna <vskrishn@codeaurora.org>