target/ppc: Move else branch to avoid large if block in mmu6xx_get_physical_address()

target/ppc: Move else branch to avoid large if block in mmu6xx_get_physical_address()

In mmu6xx_get_physical_address() we have a large if block with a two
line else branch that effectively returns. Invert the condition and
move the else there to allow deindenting the large if block to make
the flow easier to follow.

Reviewed-by: Nicholas Piggin <npiggin@gmail.com>
Signed-off-by: BALATON Zoltan <balaton@eik.bme.hu>
Signed-off-by: Nicholas Piggin <npiggin@gmail.com>