83xx, uec: split enet_interface in two variables

There's no sensible reason to unite speed and interface type into
one variable.  So split this variable enet_interface into two
vars: enet_interface_type, which hold the interface type and speed.

Also: add the possibility for switching between 10 and 100 MBit
interfaces on the fly, when running in FAST_ETH mode.

Signed-off-by: Heiko Schocher <hs@denx.de>
Signed-off-by: Ben Warren <biggerbadderben@gmail.com>