msm: buspm: Correct size type in buspm_alloc_params

Clients can pass unsigned long size results in
allocating large chunk of memory which intern results
in arbitary kernel read/write. This patch corrects
this value.

CRs-Fixed: 612384
Change-Id: Ia5780739ca832680ded2a2677b73d6e7f165bf99
Signed-off-by: Sana Venkat Raju <c_vsana@codeaurora.org>