iwlagn: remove ISR ops

The ISR (interrupt service routine) ops are now
no longer necessary since they are the same for
all devices this driver now handles.

Signed-off-by: Johannes Berg <johannes.berg@intel.com>
Signed-off-by: Wey-Yi Guy <wey-yi.w.guy@intel.com>
Signed-off-by: John W. Linville <linville@tuxdriver.com>