smsc: Move the SMC (SMSC) drivers

Moves the SMC (SMSC) drivers into drivers/net/ethernet/smsc/ and the
necessary Kconfig and Makefile changes.  Also did some cleanup
of NET_VENDOR_SMC Kconfig tag for the 8390 based drivers.

CC: Nicolas Pitre <nico@fluxnic.net>
CC: Donald Becker <becker@scyld.com>
CC: Erik Stahlman <erik@vt.edu>
CC: Dustin McIntire <dustin@sensoria.com>
CC: Steve Glendinning <steve.glendinning@smsc.com>
CC: David Hinds <dahinds@users.sourceforge.net>
Signed-off-by: Jeff Kirsher <jeffrey.t.kirsher@intel.com>