cfg80211: express channels with a KHz component

Some bands (S1G) define channels centered on a non-integer
MHz. Give ieee80211_channel and cfg80211_chan_def a
freq_offset component where the final frequency can be
expressed as:

MHZ_TO_KHZ(chan->center_freq) + chan->freq_offset;

Also provide some helper functions to do the frequency
conversion and test for equality.

Retain the existing interface to frequency and channel
conversion helpers, and expose new ones which handle
frequencies in units of KHz.

Some internal functions (net/wireless/chan.c) pass around
a frequency value. Convert these to units of KHz.

mesh, ibss, wext, etc. are currently ignored.

Signed-off-by: Thomas Pedersen <thomas@adapt-ip.com>
Link: https://lore.kernel.org/r/20200402011810.22947-3-thomas@adapt-ip.com


Signed-off-by: Johannes Berg <johannes.berg@intel.com>
Git-commit: 934f4c7dd3a544bb8000f7436f1f0e12e04ebc37
Git-repo: git://git.kernel.org/pub/scm/linux/kernel/git/jberg/mac80211-next.git


CRs-Fixed: 2743684
Change-Id: Ib50d4946c6edab98efd9626dbcbae5ee3d905a78
Signed-off-by: Vinita S. Maloo <vmaloo@codeaurora.org>