target/microblaze: Rename DISAS_UPDATE to DISAS_EXIT

target/microblaze: Rename DISAS_UPDATE to DISAS_EXIT

The name "update" suggests that something needs updating, but
this is not the case.  Use "exit" to emphasize that nothing
needs doing except to exit.

Reviewed-by: Philippe Mathieu-Daudé <f4bug@amsat.org>
Reviewed-by: Edgar E. Iglesias <edgar.iglesias@xilinx.com>
Tested-by: Edgar E. Iglesias <edgar.iglesias@xilinx.com>
Signed-off-by: Richard Henderson <richard.henderson@linaro.org>