[flash_ctrl] Fix the way data forwarding is handled.
[flash_ctrl] Fix the way data forwarding is handled.

- When nothing in the pipeline, forward data
- When something is in the pipeline, look at the registered results

Signed-off-by: Timothy Chen <d7b2ae30d8c0f215facd721941273067fd1d14a2@google.com>

lowRISC/opentitan,lowRISC/opentitan,lowRISC/opentitan,lowRISC/opentitan,lowRISC/opentitan,lowRISC/opentitan