sparc: fix leon3 casa instruction when MMU is disabled

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>