msm: usb_bam: Change the usb bam pipe threshold to 16 bytes

In WHCK loopback test, data transfer stall is observed on DL
where A2 could not send data to USB due to hardware buffer.
Currently the pipe event threshold of A2 BAM is 16 bytes. So
A2 BAM will notify USB BAM almost as soon as it has any new
data. But the pipe event threshold of USB BAM was 512 bytes.
Change the usb bam pipe threshold to 16 bytes, so USB BAM
can tell A2 BAM as soon as USB BAM consumes a descriptor with
even small data trunk.

Change-Id: I2703b8fab4260d5da5366d0a0d9b5a309d11a7a4
CRs-Fixed: 374584
Acked-by: Amit Blay <ablay@codeaurora.org>
Signed-off-by: Rohit Vaswani <rvaswani@codeaurora.org>