xtensa: userspace: fix uninitialized return values in mpu_map_region_add

[agiardin]

When CONFIG_XTENSA_MPU_ONLY_SOC_RANGES is enabled, the first region added to the MPU table is added to an empty table. mpu_map_region_add has a special section for handling this case, but it does not update the value of *first_idx which is returned to the caller. This results in returning a garbage value to the caller.

[github-actions[bot]]

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