usb_dc_dw: Correctly assign endpoint FIFOs

The designware hardware in dedicated FIFO mode (which is all we
support right now for lack of shared-FIFO hardware) has one hardware
FIFO per IN (i.e. transmit) endpoint.  But it doesn't assign them on
its own, it's the drivers responsibility to populate the TxFNum field
of the DIEPCTL registers with integer indices corresponding to the
desired FIFO.

We weren't doing that, which meant that all IN endpoints were sharing
the same FIFO zero which is supposed to be dedicated to EP0 control
transfers.  The net effect is that sometimes outbound transfers would
be corrupted, showing data from the wrong endpoint.  More often that
not this would leak from control transfers over to the
higher-bandwidth bulk endpoints of the application, but occasionally
you'd see a control transfer itself get borked and the USB device
would glitch.

Get this right and set the FIFOs properly.

Signed-off-by: Andy Ross <andrew.j.ross@intel.com>