ath5k: remove the use of SWI interrupt

We don't need to generate a software interrupt (SWI) just to schedule a tasklet
- we can just schedule the tasklet directly.

Rename constants, names, etc to reflect the fact that we don't use SWI any more.

Also move the flag handling into the tasklet and prepare it to behave correctly
when there are multiple flags present.

Signed-off-by: Bruno Randolf <br1@einfach.org>
Signed-off-by: John W. Linville <linville@tuxdriver.com>