nosy_start/stop_snoop() are always only called by the ioctl method, i.e.
with IRQs enabled.  packet_handler() and bus_reset_handler() are always
only called by the IRQ handler.  Hence neither one needs to track IRQ
flags.

To underline the call context of packet_handler() and
bus_reset_handler(), rename these functions to *_irq_handler().
Signed-off-by: NStefan Richter <stefanr@s5r6.in-berlin.de>