target/arm: Correctly bound length in sve_zcr_get_valid_len

target/arm: Correctly bound length in sve_zcr_get_valid_len

Currently, our only caller is sve_zcr_len_for_el, which has
already masked the length extracted from ZCR_ELx, so the
masking done here is a nop.  But we will shortly have uses
from other locations, where the length will be unmasked.

Saturate the length to ARM_MAX_VQ instead of truncating to
the low 4 bits.

Signed-off-by: Richard Henderson <richard.henderson@linaro.org>
Reviewed-by: Peter Maydell <peter.maydell@linaro.org>
Message-id: 20210723203344.968563-2-richard.henderson@linaro.org
Signed-off-by: Peter Maydell <peter.maydell@linaro.org>