dmm/metex14: unbreak packet request helper return code

dmm/metex14: unbreak packet request helper return code

Return SR_OK in case of successful transmission of a packet request. The
previous implementation passed the serial layer's verbatim return value,
which was non-negative non-null (read: above zero) in case of success,
which is none of the expected return codes of a packet request routine.

This amends commit 379e95c587e1d and completes the adjustment which was
started in commit a4be2b327be8. The issue has gone unnoticed in the past
since it took not effect. The serial-dmm caller only tested for negative
return values.