local/net: bcm4358: fix panic when unregistering p2p interface

local/net: bcm4358: fix panic when unregistering p2p interface

When unregistering p2p interface, following kernel panic occurs:

   BUG: failure at mm/slub.c:3413/kfree()!
   Kernel panic - not syncing: BUG!
   CPU: 1 PID: 2698 Comm: wpa_supplicant Not tainted 4.1.36-arm64-tm2 #1
   ...
   [<ffffffc000a6bc78>] panic+0xf0/0x234
   [<ffffffc00019cc1c>] show_slab_objects+0x0/0x264
   [<ffffffc0009e2204>] nl80211_set_wowlan+0x748/0x81c
   ...

It is because kfree(wdev->wiphy->wowlan_config) is called from
cfg80211_rdev_free_wowlan(), but it is assigned with const struct
pointer from the commit c1cdee9f047f ("LOCAL / net: bcm4358: set
wowlan_config for broadcom driver"). So this patch fixes to assign
it with kmemdup.

Change-Id: Ia924d27d7f262dfd0b49d61ee0214f2ae24fb5c7
Signed-off-by: Seung-Woo Kim <sw0312.kim@samsung.com>