drivers/block/swim.c: remove unnecessary platform_set_drvdata()

The driver core clears the driver data to NULL after device_release or
on probe failure.  Thus, it is not needed to manually clear the device
driver data to NULL.

Signed-off-by: Jingoo Han <jg1.han@samsung.com>
Cc: Jean Delvare <khali@linux-fr.org>
Cc: Jens Axboe <axboe@kernel.dk>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>