ieee80211: share 802.11 unit conversion helpers

MHZ_TO_KHZ, and KHZ_TO_MHZ are useful to drivers and
elsewhere so export these in the common ieee80211 header.
Move the power helpers also because we might as well.

Signed-off-by: Thomas Pedersen <thomas@adapt-ip.com>
Link: https://lore.kernel.org/r/20200402011810.22947-2-thomas@adapt-ip.com
Signed-off-by: Johannes Berg <johannes.berg@intel.com>