zd1211rw: support for mesh interface and beaconing

The previously unused CR_CAM_MODE register is set to MODE_AP_WDS. This makes the
driver ack mesh (WDS) frames. It does not affect Infra functionality of the
driver.

Previously missing beaconing support has been added. This might also help
implement a currently missing ah-hoc mode.

Support for interrupts from the device have been added, but we are not handling
most of them.

Mesh interfaces are considered associated as long as the interface is up.

Signed-off-by: Luis Carlos Cobo <luisca@cozybit.com>
Signed-off-by: John W. Linville <linville@tuxdriver.com>