hw/intc/arm_gicv3_its: Refactor process_its_cmd() to reduce nesting

hw/intc/arm_gicv3_its: Refactor process_its_cmd() to reduce nesting

Refactor process_its_cmd() so that it consistently uses
the structure
  do thing;
  if (error condition) {
      return early;
  }
  do next thing;

rather than doing some of the work nested inside if (not error)
code blocks.

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: 20220111171048.3545974-8-peter.maydell@linaro.org