wifi: ieee80211: split mesh definitions out

The ieee80211.h file has gotten very long, start splitting it
by putting mesh definitions into a separate file.

type=cleanup
ticket=none

Change-Id: I3befb4bf6ace0315758a1794224ddd18c4652e32
Signed-off-by: Johannes Berg <johannes.berg@intel.com>
Reviewed-on: https://gerritwcs.ir.intel.com/c/iwlwifi-stack-dev/+/228837
tested: iil_jenkins iil_jenkins <EC.GER.UNIX.IIL.JENKINS@INTEL.COM>
Reviewed-by: Miriam Rachel Korenblit <miriam.rachel.korenblit@intel.com>
x-iwlwifi-stack-dev: 966d188d67ce8228a0936fbb0ab0c47dc479c42c
4 files changed