bcm2835-mmc: Deduplicate reset of driver data on remove

The BCM2835 MMC host driver sets the device's driver data pointer to
NULL on ->remove() even though the driver core subsequently does the
same in __device_release_driver().  Drop the duplicate assignment.

Signed-off-by: Lukas Wunner <lukas@wunner.de>
Cc: Frank Pavlic <f.pavlic@kunbus.de>