mwl8k: enable multi-BSS AP operation

mwl8k: enable multi-BSS AP operation

As follows:
- GET_HW_SPEC is now responsible for setting
  priv->{ap,sta}_macids_supported, which are bitmasks of supported
  macids for AP and STA mode.  (Typically, STA firmware images will
  support only one macid, #0, in STA mode, and AP firmware images
  will support macids #0-7, in AP mode.)
- Our wiphy ->interfaces_modes is now set based on the non-zero-ness
  of these two bitmasks.
- We main priv->macids_used, a bitmask of which macids are currently
  in use.  ->add_interface() will assign the lowest free macid for
  this interface type as it is created, or bail out if there are no
  more free macids to assign.  ->delete_interface() will mark the
  macid as being free again.

This enables the multi-BSS code added in the previous commits.

Signed-off-by: Lennert Buytenhek <buytenh@marvell.com>
Signed-off-by: John W. Linville <linville@tuxdriver.com>