wifi: mac80211_hwsim: use 32-bit skb cookie

We won't really have enough skbs to need a 64-bit cookie,
and on 32-bit platforms storing the 64-bit cookie into the
void *rate_driver_data doesn't work anyway. Switch back to
using just a 32-bit cookie and uintptr_t for the type to
avoid compiler warnings about all this.

Fixes: 4ee186fa ("wifi: mac80211_hwsim: fix race condition in pending packet")
Signed-off-by: Johannes Berg <johannes.berg@intel.com>