usb: u_bam: Reset pending pkt counter when closing dmux channel

bam dmux driver will take care of freeing all pending skbs while
closing the channel. Reset packets pending with bam dmux to zero
to reflect the same.

Change-Id: Ibdb35a11166399b2691ceec567254f3bcf8512b3
Signed-off-by: Vamsi Krishna <vskrishn@codeaurora.org>