wcnss: Fix buffer overflow in wcnss_prealloc_get

There is potential integer truncation in the wcnss_prealloc_get api.
size_t is 8 byte on x64 platform and "unsigned int" is 4 byte.
To avoid this integer truncation, pass size as size_t instead
of unsigned int.

CRs-Fixed: 2269610
Change-Id: I14b274dd7cad98b55fdce1aaa27783272231afde
Signed-off-by: Anurag Chouhan <achouhan@codeaurora.org>