USB: gadget: u_bam: Add module parameter for max packets pending with bam

Add module parameter for configuring maximum number of packets to be
pushed to bam without ack from bam dmux. Also add counters for number
of times rx flow control enabled and disabled, which will be helpful
for debugging.

Change-Id: Iefad2c923534b92ca729def85a36ef787b790690
Signed-off-by: Vijayavardhan Vennapusa <vvreddy@codeaurora.org>