drivers/char/riscom8: clean up irq handling

Make irq handling more efficient, by passing board pointer via
request_irq() to our irq handler's dev_id argument.

This eliminates a table lookup upon each interrupt, and eliminates an
associated global variable (the table).

Signed-off-by: Jeff Garzik <jgarzik@redhat.com>

diff --cc drivers/char/riscom8.c
Simple merge