usb: rmnet: Use buffer levels as to flow control

usb bam mux driver enables flow control based on number of packets
pending by mux. Instead enable/disable flow control based internal
buffer levels to prevent buffer overflows.

Change-Id: I6617cebaec2d6b7d0c3d903f391cdc90f1ce83b3
Signed-off-by: Vamsi Krishna <vskrishn@codeaurora.org>