nl80211: include frequency offset in survey info

Recently channels gained a potential frequency offset, so
include this in the per-channel survey info.

Signed-off-by: Thomas Pedersen <thomas@adapt-ip.com>
Link: https://lore.kernel.org/r/20200922022818.15855-16-thomas@adapt-ip.com


[add the offset only if non-zero]
Signed-off-by: Johannes Berg <johannes.berg@intel.com>