cfg80211: use __force __rcu to suppress sparse warning

cfg80211: use __force __rcu to suppress sparse warning

The code assigns a constant value (a pointer to a static variable)
to an RCU pointer, which results in a sparse warning:
  reg.c:112:10: warning: cast adds address space to expression (<asn:4>)

Suppress this warning by using __force.

Signed-off-by: Johannes Berg <johannes.berg@intel.com>