[SUNSAB]: Fix several bugs.

	* don't register irq until ->startup() (and release in ->shutdown()).
That avoids oopsen with the current tree when interrupt comes before we'd
set up the data structures for ttyb.
	* handle console=ttyS... even when OBP talks to screen/keyboard
	* register irq handler for each port, let kernel/irq/handle.c
call it for both if needed.  Kills code duplication in sunsab_interrupt().
BTW, there'd been bitrot in it - ttya handling had stopped calling
check_status() on BRK (correctly), ttyb copy of that code had kept the
bogus call in that case.

Signed-off-by: Al Viro <viro@zeniv.linux.org.uk>
Signed-off-by: David S. Miller <davem@davemloft.net>