Blackfin: redo handling of bad irqs

With the common IRQ code initializing much more of the irq_desc state, we
can't blindly initialize it ourselves to the local bad_irq state.  If we
do, we end up wrongly clobbering many fields.  So punt most of the bad irq
code as the common layers will handle the default state, and simply call
handle_bad_irq() directly when the IRQ we are processing is invalid.

Signed-off-by: Mike Frysinger <vapier@gentoo.org>