[PATCH, take 2] libertas: rework event subscription

[Holger Schurig]

> * fix a bug where it was not possible to disable an event

> The old code also contained this bug:
>
> $ cd /sys/kernel/debug/lbs_wireless/eth1
> $ cat failure_count
> 0 0 0
> $ echo 60 1 1 >failure_count
> $ cat failure_count
> 60 1 1
> $ echo 0 0 0 >failure_count
> $ cat failure_count
> 0 0 1
>
> The new code turns off the proper event bitmask as well, so
> the last "cat" returns "0 0 0".

With "old code" I didn't mean my take 1, I mean the code that is 
right now in drivers/net/wireless/libertas/debugfs.c