cfg80211: reject channels/chandefs with KHz offset >= 1000

This should be covered by the next MHz, make sure that the
numbers are always normalized.

Signed-off-by: Johannes Berg <johannes.berg@intel.com>
Link: https://lore.kernel.org/r/20200424120103.12b91ecf75f9.I4bf499d58404283bbfacb517d614a816763bccf2@changeid


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