drivers: usb_dc_rpi_pico: set AVAILABLE bit separately

The DPSRAM ports can run at different clocks, this is the default
configuration, follow the advice in the datasheet and wait 3 nop
instructions before setting the AVAILABLE bit.

It can be observed that when the controller is continuously sending data
to the host, it rarely has a 0-byte transaction instead of a short
packet. The reason for this is not easy to find, it also seems to depend
on the runtime of individual components. This may fix the problem, but
there is no sure proof that this is the solution.

Signed-off-by: Johann Fischer <johann.fischer@nordicsemi.no>