net: ethernet: remove unnecessary platform_set_drvdata()

The driver core clears the driver data to NULL after device_release
or on probe failure, since commit 0998d0631001288a5974afc0b2a5f568bcdecb4d
(device-core: Ensure drvdata = NULL when no driver is bound).
Thus, it is not needed to manually clear the device driver data to NULL.

Signed-off-by: Jingoo Han <jg1.han@samsung.com>
Acked-by: Nicolas Ferre <nicolas.ferre@atmel.com>
Acked-by: Rob Herring <rob.herring@calxeda.com>
Acked-by: Roland Stigge <stigge@antcom.de>
Acked-by: Mugunthan V N <mugunthanvnm@ti.com>
Reviewed-by: H Hartley Sweeten <hsweeten@visionengravers.com>
Tested-by: Roland Stigge <stigge@antcom.de>
Signed-off-by: David S. Miller <davem@davemloft.net>