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

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

Fix process_its_cmd() 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: 20220111171048.3545974-7-peter.maydell@linaro.org