hw/intc/arm_gicv3_its: Split error checks

In most of the ITS command processing, we check different error
possibilities one at a time and log them appropriately. In
process_mapti() and process_mapd() we have code which checks
multiple error cases at once, which means the logging is less
specific than it could be. Split those cases up.

Signed-off-by: Peter Maydell <peter.maydell@linaro.org>
Reviewed-by: Richard Henderson <richard.henderson@linaro.org>
Message-id: 20220201193207.2771604-14-peter.maydell@linaro.org