wimax: generic device management (registration, deregistration, lookup)

Implements the basic life cycles of a 'struct wimax_dev', some common
generic netlink functionality for marshalling calls to user space,
and the device state machine.

For looking up net devices based on their generic netlink family IDs,
use a low overhead method that optimizes for the case where most
systems have a single WiMAX device, or at most, a very low number of
WiMAX adaptors.

Signed-off-by: Inaky Perez-Gonzalez <inaky@linux.intel.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@suse.de>