wcnss: Pre-alloc memory for WLAN driver

WLAN driver will use this pre-allocated memory (when available) for
large memory allocations; this will prevent WLAN driver load
failures because of the un-availability of the large size slabs
during module load.

Change-Id: I8a8139bdf343ddc871036f6d5c6ab90993816de0
Signed-off-by: Sameer Thalappil <sameert@codeaurora.org>