rp2350/pad_checksum: fix set cpu

[AnnsAnns]

See #2334

This PR should hopefully fix the incorrect cpu for the rp2350 in the pad_checksum function.

[lurch]

Perhaps this "and checksummed" file-comment is also misleading, as there's a code-comment a few lines above saying "No CRC..." ?
Also, looks like the --seed argument is entirely unused by the RP2350 version of this script? 🤔 (Ahhh, but perhaps it's needed for CLI-compatibility with the RP2040 version)

[AnnsAnns]

After further investigation, it appears like this entire script is completely misleading, based on chapter 5.9.5 of the RP2350 documentation the boot2 checksum is not needed for the rp2350:

Unlike RP2040, there is no requirement for flash binaries to have a checksummed "boot2" flash setup function at flash address 0. The RP2350 bootrom performs a simple best-effort XIP setup during flash scanning, and a flash-resident program can continue executing in this state, or can choose to reconfigure the QSPI interface at a later time for best performance.

I am not really sure how to proceed with this PR, wouldn't it make sense to completely get rid of that script based on the fact that this isn't something the pico2 actually needs?