sparc: fix leon3 casa instruction when MMU is disabled

Since the commit af7a06ba:
`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: NKONRAD Frederic <frederic.konrad@adacore.com>
Signed-off-by: NMark Cave-Ayland <mark.cave-ayland@ilande.co.uk>

target/sparc/translate.c

@@ -2093,6 +2093,11 @@ static DisasASI get_asi(DisasContext *dc, int insn, TCGMemOp memop)
             type = GET_ASI_BFILL;
             break;
         }
+
+        /* MMU_PHYS_IDX is used when the MMU is disabled to passthrough the
+         * permissions check in get_physical_address(..).
+         */
+        mem_idx = (dc->mem_idx == MMU_PHYS_IDX) ? MMU_PHYS_IDX : mem_idx;
     } else {
         gen_exception(dc, TT_PRIV_INSN);
         type = GET_ASI_EXCP;