USB: u_ether: Submit requests from rx_complete until limit reached

RX complete (completion callback of RX reqs) defers processing of
received skbs and requeuing of RX requests to workqueue. Also, in
workqueue function it pushes skb to network first and then queues
expty RX requests to hardware.
During high throughput or stressed testing there is possiblity of
USB hardware running out of RX requests for a while due to
delayed workqueue scheduling. This affects TCP TX (downlink) as
TCP ACKs for DL traffic are sent from PC to device (or RX for
u_ether). To workaround this queues RX requests from rx_complete
itself until pending skbs for network stack (that are still
processed by wq) reach a limit defined by a newly introduced
module param: 'u_ether_rx_pending_thl'. Its default value is 500.

Change-Id: I83b355d3dd3989cc425663e5b219c45661904f75
Signed-off-by: Manu Gautam <mgautam@codeaurora.org>
Signed-off-by: Ajay Agarwal <ajaya@codeaurora.org>