msm: mhi: Check bb ring and transfer ring when checking for space

When checking for available spaces, check available spaces on
both bounce buffer ring and transfer ring and return min.

Change-Id: I9208b46c32821de3f5d9e3d828087d7bc29b9546
CRs-Fixed: 1055681
Signed-off-by: Sujeev Dias <sdias@codeaurora.org>