get_valid_reuse_factors returns some invalid RFs for the im2col conv impementations

[jmitrevs]

Prerequisites

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• Test that the bug appears on the current version of the master branch. Make sure to include the commit hash of the commit you checked out.
• Check that the issue hasn't already been reported, by checking the currently open issues.
• If there are steps to reproduce the problem, make sure to write them down below.
• If relevant, please include the hls4ml project files, which were created directly before and/or after the bug.

Quick summary

The parallel resource conv implementations (at least for Vivado and Vitis) require RF <= FILT_HEIGHT * FILT_WIDTH * N_CHAN, but get_valid_reuse_factors allows for larger reuse factors.

Steps to Reproduce

One can try setting a large RF, and the project is created with the large RF, but then this triggers an assertion error.

Expected behavior

The RF should be set to the maximum legal value