cfg80211: Add support to configure a beacon data rate

This allows an option to configure a single beacon tx rate for an AP.

Signed-off-by: Purushottam Kushwaha <pkushwah@qti.qualcomm.com>
Signed-off-by: Johannes Berg <johannes.berg@intel.com>
Git-repo: git://git.kernel.org/pub/scm/linux/kernel/git/jberg/mac80211-next.git


Git-commit: a7c7fbff6a408d00431c705bbe3dfc5f51e3f1c4
CRs-fixed: 1025311
Change-Id: I9d7ada3f573e31d4693c39df32c1aacd928f4b5f
Signed-off-by: Purushottam Kushwaha <pkushwah@codeaurora.org>