usb: gadget: rmnet: Use separate spinlock for RX and TX data paths

Current USB tethered RMNET gadget driver uses same spinlock
for both RX and TX data traffic which hits the overall performance
because of CPU contentions between RX and TX data paths. Hence use
separate spinlocks for RX and TX data traffic.

Change-Id: I128bd1aea8d08cbcf7a8ba12799846a4b06c2f1d
Signed-off-by: Hemant Kumar <hemantk@codeaurora.org>