MIPS: RB532: Use driver_data instead of platform_data

As the korina ethernet driver uses platform_get_drvdata() to extract the
driver specific data from the platform device, driver_data has to be
used here.

Signed-off-by: Phil Sutter <n0-1@freewrt.org>
Signed-off-by: Ralf Baechle <ralf@linux-mips.org>