msm: msm_bus: Explicitly clear client requests before removal

Clients can potentially leave non-zero requests before
unregistering. Such requests persist and cause instability
and severe power and performance impact.

Protect the system against such clients by explicitly
clearing the client requests before unregistration.

Change-Id: I6a905b097de41af168b5f5048b682ba995b581c2
CRs-Fixed: 407057
Signed-off-by: Gagan Mac <gmac@codeaurora.org>