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When a pipe is aborted the EHCI driver removes any pending transactions and
aborts the transfer, the XHCI driver does not but the NetBSD version does
abort the transfer from version 1.28.2.19.

Added xhci_abort_xfer modified from NetBSD xhci.c 1.28.2.19 and made
xhci_device_bulk_abort use it.

This stops SerialUSB locking up the machine when closing a device which is
still transferring data when it is closed.

Tested on a Raspberry Pi 4.

Version 0.31. Tagged as 'XHCIDriver-0_31'