openrisc: Cleanup switch code and comments

The saving of the r12 register was there for a compiler bug referring
to a port that was never upstreamed.  It should be safe to use this
as the new compiler is what we use and the old deprecated.

Also, clean up some typos and references to old names in the switch
comments.

Signed-off-by: Stafford Horne <shorne@gmail.com>