nl80211: Fix backport of struct wiphy_vendor_command

Change-Id I5bdab7e0dce095f2d846687343f78cc0dd60aab1 ("nl80211: vendor
command support") was used to backport nl80211 vendor command support
from 3.14.  Unfortunately this backport has a typo in the definition
of struct wiphy_vendor_command, namely the third param to the doit
callback function was declared as "void *data" instead of the correct
"const void *data".  Correct this flaw in the backport.

Change-Id: I1caf72215ebea94f6f9e7a930ab54d67ec8c2538
CRs-Fixed: 768724
Signed-off-by: Jeff Johnson <jjohnson@codeaurora.org>