ath5k: don't enable MSI, we cannot handle it yet

MSI is a nice thing, but we cannot enable it without changing the
interrupt handler.  If we do it, we break MSI capable hardware,
specifically AR5006 chipset.

Signed-off-by: Pavel Roskin <proski@gnu.org>
Acked-by: Nick Kossifidis <mickflemm@gmail.com>
Cc: stable <stable@kernel.org>
Signed-off-by: John W. Linville <linville@tuxdriver.com>