usb_bam: Set default BAM type as DWC3 if not specified

'Commit 8db85a3d ("platform: msm: Add USB BAM support to
ChipIdea/RMNET")' mandates BAM type to be present in the dtsi
file. Absence of this field leads to BAM probe failure.

Fix this by setting the default BAM type to DWC3 if the field
is not available.

Change-Id: I3970855e1d3682901b7557512c68bad1f77260a1
Signed-off-by: Udipto Goswami <ugoswami@codeaurora.org>