qcacmn: Fix Integer Overflow Leading to Buffer Overflow

wmi_buf_alloc() API expects length to be passed of type
uint16_t. However, the callers pass uint32_t to it.
This might result in overflow and illegal memory access
thereafter. The fix is to modify the API signature accordingly.

Change-Id: If09da4978d421269b884f7d3c933c49c81651475
CRs-Fixed: 2218346