sparc64: Fix irq_of_parse_and_map() and irq_dispose_mapping().

Stephen Rothwell noticed that I committed an earlier version
of the patch that didn't have two things fixed:

1) irq_of_parse_and_map() should return "unsigned int" not "int"
   and it should return zero for "no irq"

2) irq_dispose_mapping() should be an inline function, not a macro,
   for type checking

With feedback and suggestions from Anton Vorontsov.

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