staging: vt6656: refactor power save operation

staging: vt6656: refactor power save operation

At present the power save wake uses the listening interval and
the slow path to wake up.

The following using a beacon interval of 100 and
listen interval of 5.

The TBTT set at 100 wake-up sequence;
100 TBTT wake-up set to listen interval.
200 TBTT
300 TBTT
400 TBTT --> call vnt_next_tbtt_wakeup on slow path
	Beacon heard and passed through at the approx 500 interval.
500 TBTT
600 TBTT wake-up set to listen interval

The TBTT set at 500 wake-up sequence and always listen flagged on;
100 No TBTT
200 No TBTT
300 No TBTT
400 No TBTT
500 TBTT - beacon heard and passed through
600 No TBTT

A further enhancement because the TBTT is more precise
the dtim_period can be used instead.

When Power save is off the TBTT continues to run at the listen
interval but all the other beacons are passed.

The code in vnt_int_process_data is no longer required.

Signed-off-by: Malcolm Priestley <tvboxspy@gmail.com>
Link: https://lore.kernel.org/r/5a188bd8-7049-8063-f24d-96768ce9a6ed@gmail.com
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>