wifi: nl80211: refactor parsing CSA offsets

The CSA offset parsing happens the same way for all of
beacon template offsets, probe response template offsets
and TX offsets (for using during probe response TX from
userspace directly).

Refactor the parsing here. There's an additional check
this introduces, which is that the number of counters in
TX offsets doesn't exceed the driver capability, but as
only two counters are used at most for anything, this is
hopefully OK.

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