sparc: fix leon3 casa instruction when MMU is disabled
    
    Since the commit af7a06bac7d3abb2da48ef3277d2a415772d2ae8:
    `casa [..](10), .., ..` (and probably others alternate space instructions)
    triggers a data access exception when the MMU is disabled.
    
    When we enter get_asi(...) dc->mem_idx is set to MMU_PHYS_IDX when the MMU
    is disabled. Just keep mem_idx unchanged in this case so we passthrough the
    MMU when it is disabled.
    
    Signed-off-by: KONRAD Frederic <frederic.konrad@adacore.com>
    Signed-off-by: Mark Cave-Ayland <mark.cave-ayland@ilande.co.uk>
    (cherry picked from commit 6e10f37c86068e35151f982c976a85f1bec07ef2)
    Signed-off-by: Michael Roth <mdroth@linux.vnet.ibm.com>