[PATCH] powerpc: return to OF via trap, not exit

Do not call prom exit prom_panic. It clears the screen and the exit
message is lost.

On some (or all?) pmacs it causes another crash when OF tries to print
the date and time in its banner.

Set of_platform earlier to catch more prom_panic() calls.

Signed-off-by: Olaf Hering <olh@suse.de>
Acked-by: Segher Boessenkool <segher@kernel.crashing.org>
Signed-off-by: Paul Mackerras <paulus@samba.org>