target/arm: Rearrange disabled check for watchpoints

target/arm: Rearrange disabled check for watchpoints

Coverity rightly notes that ctz32(bas) on 0 will return 32,
which makes the len calculation a BAD_SHIFT.

A value of 0 in DBGWCR<n>_EL1.BAS is reserved.  Simply move
the existing check we have for this case.

Reported-by: Coverity (CID 1421964)
Signed-off-by: Richard Henderson <richard.henderson@linaro.org>
Reviewed-by: Philippe Mathieu-Daudé <philmd@redhat.com>
Message-id: 20200320160622.8040-2-richard.henderson@linaro.org
Reviewed-by: Peter Maydell <peter.maydell@linaro.org>
Signed-off-by: Peter Maydell <peter.maydell@linaro.org>