Multi-AP: Don't reject backhaul STA on fronthaul BSS

Multi-AP: Don't reject backhaul STA on fronthaul BSS

The Multi-AP specification only specifies that information elements have
to be added to the Association Request and Association Response frame;
it doesn't specify anything about what should be done in case they are
missing. Previously, we rejected non-backhaul associations on a
backhaul-only BSS, and non-fronthaul associations on a fronthaul-only
BSS.

However, this makes WPS fail when fronthaul and backhaul are separate
SSIDs. Indeed, WPS for the backhaul link is performed on the *fronthaul*
SSID. Thus, the Association Request frmae used for WPS *will* contain
the Multi-AP IE indicating a backhaul STA. Rejecting that association
makes WPS fail.

Therefore, accept a multi-AP backhaul STA Association Request frame on a
fronthaul-only BSS. Still issue a warning about it, but only at level
DEBUG intead of INFO. Also change the condition checking to make it
clearer.

While we're at it, also fix the handling of unexpected bits in the
Multi-AP IE. 4 bits are reserved in the specification, so these
certainly have to be ignored. The specification also doesn't say that
setting one of the other bits is not allowed. Therefore, only report
unexpected values in the Multi-AP IE, don't reject because of it. Note
that a malformed IE (containing more than one byte) still triggers a
rejection.

Signed-off-by: Arnout Vandecappelle (Essensium/Mind) <arnout@mind.be>