iwlwifi: don't handle masked interrupt

This can lead to a panic if the driver isn't ready to
handle them. Since our interrupt line is shared, we can get
an interrupt at any time (and CONFIG_DEBUG_SHIRQ checks
that even when the interrupt is being freed).

If the op_mode has gone away, we musn't call it. To avoid
this the transport disables the interrupts when the hw is
stopped and the op_mode is leaving.
If there is an event that would cause an interrupt the INTA
register is updated regardless of the enablement of the
interrupts: even if the interrupts are disabled, the INTA
will be changed, but the device won't issue an interrupt.
But the ISR can be called at any time, so we ought ignore
the value in the INTA otherwise we can call the op_mode
after it was freed.

I found this bug when the op_mode_start failed, and called
iwl_trans_stop_hw(trans, true). Then I played with the
RFKILL button, and removed the module.
While removing the module, the IRQ is freed, and the ISR is
called (CONFIG_DEBUG_SHIRQ enabled). Panic.

Cc: stable@vger.kernel.org
Signed-off-by: Emmanuel Grumbach <emmanuel.grumbach@intel.com>
Reviewed-by: Gregory Greenman <gregory.greenman@intel.com>
Signed-off-by: Johannes Berg <johannes.berg@intel.com>