soc: qcom: socinfo: Add check for subtype id

There's an overflow when subtype id is greater than 3 in current
design. Add a check to ensure returning correct value.

Change-Id: Ie8d234792f7370c95782a86e16affd383ed53c18
Signed-off-by: Zhenhua Huang <zhenhuah@codeaurora.org>