drivers: udc_dwc2: Allow larger TxFIFO writes

DMA is capable of packetizing transfers on its own and the driver
doesn't need to check available TxFIFO size when scheduling DMA
transfer.

Split packets on DWORD boundary when writing more than one packet to
TxFIFO in completer mode.

Signed-off-by: Tomasz Moń <tomasz.mon@nordicsemi.no>