Fixes error seen in #1389

Somewhat recently, during #1200, while fixing a related bug, I altered the documentation to reflect what was then the behavior of the SPI Engine for the Sleep instruction: it slept for t prescaler ticks.

What I didn't realize back then was that the intended behavior, as seen on the first SPI Engine commit, is to sleep for t+1 ticks, and that there had been a regression in the meantime.

To illustrate, I've made some simple tests, which call the sleep instruction with a parameter of t=4. Expected behavior is for it to sleep for t+1 ticks, as seen on first commit and as has been documented in the old wiki.

This is with the current main (counts only t ticks):

This is how it was before #1200
(before I changed the doc), it counts each tick as double the duration - that was the bug being solved, but is not relevant to the current PR. At this point, there had already been a regression (counts only t ticks):

This is the behavior on the first commit for the SPI Engine (counts the correct number of t+1 ticks):

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