[smscusb] Allow for alternative PHY register layouts
The LAN78xx PHY interrupt source and mask registers do not match those
used by the SMSC75xx and SMSC95xx.
Signed-off-by: Michael Brown <mcb30@ipxe.org>
[smscusb] Abstract out common SMSC USB device functionality
The smsc75xx and smsc95xx drivers include a substantial amount of
identical functionality, varying only in the base address of register
sets. Abstract out this common functionality to allow code to be
shared between the drivers.
Signed-off-by: Michael Brown <mcb30@ipxe.org>