Add support for Microchop (ex SMSC) LAN78xx usb ethernet, as used in
LAN7515 chip on Raspberry Pi 3B+.  Also tweak performance a bit
for SMSC ethernet as used in earlier Raspberry Pi models.

Note: this patch incorporates all changes from usbether-rpi and usb-ether-cdc
and therefore supersedes those two patches.  It may be necessary to revert
them before applying this patch.