rp2350: PIO-USB runs the pico-sdk stock 150 MHz

Closes the sys-clock question. rp2350: the dynamic 156 MHz switch is
removed - 150 MHz soak-tested clean (and a runtime switch truncates ETM
capture on the trace carrier). rp2040 keeps its existing 120 MHz: soak
sweeps show 120 = 8/8, stock 125 = 0/3 (bulk-OUT collapses, device NAKs
~600:1 with zero CRC errors on the wire), and 132 = 2/10 flaky despite an
exact 12n/375k divider - no divider criterion predicts rp2040 PIO-USB
health (the RX state machine samples at raw sysclk), so only
soak-validated clocks ship.