usb_bam: Get source bam size to map into USB CB

For USB QDSS BAM2BAM usecase when USB is using SMMU S1 functionality,
it is required to map QDSS BAM device register address space into
USB context bank. Currently USB BAM device register address space is
used instead of QDSS BAM device which results into garbage data with
this usecase although QDSS is not generating any data. Fix this issue
by mapping QDSS BAM device register address space.

Change-Id: I66c46513865a0acf21c28bba242845e52fbd7d67
Signed-off-by: Mayank Rana <mrana@codeaurora.org>