rp2040 PIO-USB host: second MSC test after flash reports device not mounted #4014

On the ci.lan HIL rig, the raspberry_pi_pico host on PIO-USB fails the second MSC host test after a flash with "MSC device not mounted".

Setup

• Board: raspberry_pi_pico, host on PIO-USB (-DCFG_TUH_RPI_PIO_USB=1, MinSizeRel)
• Drive: General UDisk 048d:04d2
• Probe: CMSIS-DAP E6614103E72C1D2F

Reproducer

HIL_SHUFFLE_SEED=1790578256 hil_remote.py -b raspberry_pi_pico --retry 1

This seed runs cdc_msc_hid, msc_file_explorer, msc_file_explorer_freertos, then board_test. msc_file_explorer, run second, reports "MSC device not mounted" on #4009's heads a66086e and c21f94f, and on its base df5d5e0, which has none of #4009's changes.

The CI run on c21f94f, with seed 1790570445, failed cdc_msc_hid when that test ran second. So the failure seems to follow the second MSC test after a flash, whichever test that is, rather than any one example.

Not established

• Master firmware was not run with this seed.
• No other seeds were tried.
• It is unknown whether the drive, PIO-USB timing after re-flash, or the host stack is at fault.

Found while validating #4009.