firmware: scmi: correct a validity check against power domain id

A power domain id on sandbox should be in the range from zero to
ARRAY_SIZE(scmi_pwdom) - 1. Correct the validity check logic.

Addresses-Coverity-ID: 467401 ("Out-of-bounds write")
Addresses-Coverity-ID: 467405 ("Out-of-bounds read")
Signed-off-by: AKASHI Takahiro <takahiro.akashi@linaro.org>