nvdec: Check flow return of the only current handle_frame() to fix seeking issue

nvdec: Check flow return of the only current handle_frame() to fix seeking issue

Due to uncleared last flow, decoding after seek was never possible
(last_ret == GST_FLOW_FLUSHING).
nvdec dose not need to keep track of the previous flow return,
and actually the interest is data/even flow of the current handle_frame().