ath5k: disable tasklets during reset

Based on a patch from Bruno Randolf, attempting useful
work while we are resetting the chip just leads to interface
lockups and bad descriptor data, and possibly DMAing to
freed buffers.  Let's suspend all tasklets while
reprogramming the registers in the card to avoid such
problems.

In the future we can convert the tasklets to threaded
interrupt handlers to simplify things.

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