target-cris: Fix buffer overflow

Report from smatch:

target-cris/translate.c:3464 cpu_dump_state(32) error:
 buffer overflow 'env->sregs' 4 <= 255

sregs is declared 'uint32_t sregs[4][16]', so the first index must be
less than 4 or ARRAY_SIZE(env->sregs).

Signed-off-by: Stefan Weil <sw@weilnetz.de>
