uqmi: use correct value for connection checking

uqmi: use correct value for connection checking

Originally, the implementation only checked if uqmi command
execution succeeded properly without actually checking it's returned data.

This lead to a pass, even when the returned data was indicating an error.

Rework the verification to actually check the returned data,
which can only be correct if the uqmi command itself also executed correctly.

On command execution success, value "pdh_" is a pure numeric value.

Signed-off-by: Koen Vandeputte <koen.vandeputte@ncentric.com>