drivers: usb_dc_rpi_pico: avoid infinite unhandled irq retriggers

This driver enables a number of interrupts it does not attempt to handle.
This results in "unhandled IRQ: 0x...." messages being printed, and the
interrupt handler retriggers immediately again, and this happens again
and again forver, because nothing ends up clearing the interrupt.

This change implements very limited handling of these interrupts. A custom
warning is logged, and the interrupt is cleared.

This change does not imply that doing this is sufficient. More changes may
need to be implemented to more gracefully re-start transactions or
re-arm some endpoints, but this is one step in the right direction,
and at least the OS doesn't freeze up.

Signed-off-by: Purdea Andrei <andrei@purdea.ro>