[if_dwc] add support for multi-descriptor packets in TX path

Original if_dwc driver used m_defrag as an implementation shortcut but on
1000Mb networks it affects performance. Implement multi-descriptor support for
TX path.

Tested on RK3399-Firefly, patch adds ~15% of network throughput.

Reviewed By: manu
Differential Revision: https://reviews.freebsd.org/D27520