target/xtensa: disas/xtensa: fix coverity warnings

Coverity warnings CID 1385146, 1385148 1385149 and 1385150 point that
xtensa_opcode_num_operands and xtensa_format_num_slots may return -1
even when xtensa_opcode_decode and xtensa_format_decode succeed. In that
case unsigned counters used to iterate through operands/slots will not
do the right thing.
Make counters and loop bounds signed to fix the warnings.

Signed-off-by: Max Filippov <jcmvbkbc@gmail.com>