drivers: usb_dc_rpi_pico: starting read on transfer EPs

This commit partially reverts a change which was introduced in the
previous commit 5b9a0e54.
usb_dc_ep_start_read() should also be called on transfer endpoints
like it has been before, otherwise the endpoint will not be armed
after it has been reconfigured.

Signed-off-by: Manuel Aebischer <manuel.aebischer@netmodule.com>