iwlagn: probe would crash with DEBUG_SHIRQ

This is since my patch:

    iwlagn: introduce transport layer and implement rx_init

The IRQ is requested before the locks are initialized, hence the crash.
Initialize the tasklet before we request the IRQ on the way.

Reported-by: Johannes Berg <johannes.berg@intel.com>
Tested-by: Johannes Berg <johannes.berg@intel.com>
Signed-off-by: Emmanuel Grumbach <emmanuel.grumbach@intel.com>
Signed-off-by: Wey-Yi Guy <wey-yi.w.guy@intel.com>