ath5k: disable MIB interrupts

The MIB interrupt fires whenever counters overflow; however without
support for automatic noise immunity, we can sometimes get an interrupt
storm.  The get_stats() callback reads the counters anyway so we can
disable the interrupt for now until ANI is implemented.  This fixes
the issue reported in http://bugzilla.kernel.org/show_bug.cgi?id=12647

.

Changes-licensed-under: 3-Clause-BSD

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