hw/intc/arm_gicv3_its: Fix return codes in process_mapd()

hw/intc/arm_gicv3_its: Fix return codes in process_mapd()

Fix process_mapd() to consistently return CMD_STALL for memory
errors and CMD_CONTINUE for parameter errors, as we claim in the
comments that we do.

Signed-off-by: Peter Maydell <peter.maydell@linaro.org>
Reviewed-by: Alex Bennée <alex.bennee@linaro.org>
Reviewed-by: Richard Henderson <richard.henderson@linaro.org>
Message-id: 20220111171048.3545974-11-peter.maydell@linaro.org