ath5k: correct interrupt storm warning

Ben Greear points out that the "too many interrupts" message will
never print in the intended case since the interrupt counter
will be -1 after the loop.  Change it to pre-decrement so it will
be 0 on the thousandth iteration.

Cc: Ben Greear <greearb@candelatech.com>

Signed-off-by: Bob Copeland <me@bobcopeland.com>
Signed-off-by: John W. Linville <linville@tuxdriver.com>