usb: rmnet: Update flow control thresholds for bam dmux

USB driver maintains number of packets pending with bam mux driver
and check with flow control thresholds to enable or disable flow
control. Default flow control enable threshold is set to 1000 ip
packets. However bam dmux can hold only upto ~250 packets. This
is causing lot of ip packets to be dropped instead of starting the
flow control. Update the flow control enable threshold with 200 and
disable threshold with 125. Also changing module parameter names to
better reflect usb bam mux driver.

CRs-fixed: 316595
Change-Id: I28416089680fe9d578ccebc48b0f4773007d3445
Signed-off-by: Vamsi Krishna <vskrishn@codeaurora.org>