usb: gadget: f_fs: Queue request after setting is_busy flag

Currently the driver queues OUT EP request after ensuring that
is_busy flag is false, and then sets it true. It might be
possible that the queued request is completed before the
execution reaches next line to set is_busy to true. As a result,
is_busy remains true even after successful completion and no
further request is queued.
Fix this by first setting is_busy to true and then queueing the
request.

Change-Id: I87fce4e2cc94be8e6b6fb63fb1fc9afb9cf0d005
Signed-off-by: Ajay Agarwal <ajaya@codeaurora.org>