mac80211: fix a warning related to pointer conversion to u64 cookie

On a 32 bit system (in this case an omap 3430 system) gcc warned about
pointer conversion:

net/mac80211/work.c: In function 'ieee80211_remain_on_channel_timeout':
net/mac80211/work.c:534: warning: cast from pointer to integer of different size
net/mac80211/work.c: In function 'ieee80211_remain_done':
net/mac80211/work.c:1030: warning: cast from pointer to integer of different size
net/mac80211/work.c: In function 'ieee80211_wk_remain_on_channel':
net/mac80211/work.c:1056: warning: cast from pointer to integer of different size
net/mac80211/work.c: In function 'ieee80211_wk_cancel_remain_on_channel':
net/mac80211/work.c:1072: warning: cast from pointer to integer of different size

Fix it by casting the pointers to unsigned long instead. This makes the
compiler happy again.

Compile-tested only.

Cc: Johannes Berg <johannes@sipsolutions.net>
Signed-off-by: Kalle Valo <kalle.valo@nokia.com>
Signed-off-by: John W. Linville <linville@tuxdriver.com>