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- Add return values to sp_flush_stream(), sp_pass_writen(),
  sp_execute_opbuf(), sp_execute_opbuf_noflush(),
  sp_check_opbuf_usage(), sp_do_read_n().
- Use those return values to propagate errors instead of exiting.
  In some places this has to wait for core API changes (error handling for
  chip_readb, chip_readn, chip_write) hence comments are added instead.

Corresponding to flashrom svn r1719.
Signed-off-by: Stefan Tauner <stefan.tauner@alumni.tuwien.ac.at>
Acked-by: Stefan Tauner <stefan.tauner@alumni.tuwien.ac.at>