net: designware: Rename the driver var name to eth_designware

The driver variable name is eth_sandbox, which is probably a copy-paste
mistake. Fix it.

Signed-off-by: Marek Vasut <marex@denx.de>
Cc: Dinh Nguyen <dinguyen@opensource.altera.com>
Acked-by: Joe Hershberger <joe.hershberger@ni.com>