library/util_pack: Use m_axis_ready to reset accum

[podgori]

PR Description

This commit restores the idle behavior and splits it from the backpressure state which discards any partial accumulation, so that the next enabled window starts on a clean boundary.

This behavior was observed in a simulation which toggles CPACK's fifo_wr_en input; in a JESD TPL - CPACK HW chain, the fifo_wr_en input stays asserted since the jesd transfer is continuous, basically combining idle/flush when backpressured.

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)
• Documentation

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

[IstvanZsSzekely]

Both unit and system level testbenches passed.