staging: wlan-ng: get memory from kernel allocators instead of big static buffer

This patch fix the following sparse warnings in prism2fw.c:
warning: memset with byte count of 120000

Signed-off-by: Sergio Paracuellos <sergio.paracuellos@gmail.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>