[SPARC64]: Fix trap state reading for instruction_access_exception.

1) Read ASI_IMMU SFSR not ASI_DMMU.
2) IMMU has no SFAR, read TPC instead
3) Delete old and incorrect comment about the DTLB protection
   trap having a dependency on the SFSR contents in order to
   function correctly

Signed-off-by: David S. Miller <davem@davemloft.net>