Clean up DXE's IRQ handling

Free irq's on shutdown
Check for errors
Refactor and move irq allocation to dxe_init
Remove sched scan ops

Scheduled scanning is not related to normal scanning and should
not be implemented if not supported.
Merge pull request #10 from pof2/master

Some DXE improvements.
Add join implementation for connecting to BSS

Send CONFIG_BSS and JOIN before sending AUTH frame

Signed-off-by: Eugene Krasnikov <k.eugene.e@gmail.com>
Handle update scan params in proper way

Signed-off-by: Eugene Krasnikov <k.eugene.e@gmail.com>
Shift TX descriptor ring head after each packet

The TX ring head was always pointing to the first descriptor.
That's why driver could only send 1 frame.

Signed-off-by: Eugene Krasnikov <k.eugene.e@gmail.com>