SPI Engine: fix transfers with echo_sclk

[LBFFilho]

PR Description

This PR fixes echo_sclk transfers, which were broken for some cases. Notably, split transfers were problematic, affecting projects like the ad4630. Also fixes another issue where the first transfer would return 0.

tb: analogdevicesinc/testbenches#190

PR Type

• Bug fix (change that fixes an issue)
• New feature (change that adds new functionality)
• Breaking change (has dependencies in other repos or will cause CI to fail)

PR Checklist

• I have followed the code style guidelines
• I have performed a self-review of changes
• I have compiled all hdl projects and libraries affected by this PR
• I have tested in hardware affected projects, at least on relevant boards
• I have commented my code, at least hard-to-understand parts
• I have signed off all commits from this PR
• I have updated the documentation (wiki pages, ReadMe files, Copyright etc)
• I have not introduced new Warnings/Critical Warnings on compilation
• I have added new hdl testbenches or updated existing ones

[LBFFilho]

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