ps3disk: Remove superfluous cast

As ps3disk is a ppc64-only driver, sector_t equals to unsigned long, and the
cast is not needed.

Reuse in another (possibly 32-bit) driver is protected by the safety net called
`compiler warning' (with the cast, it may silently truncate to 32-bit).
If sector_t ever changes, we will get a compiler warning as well (with the
cast, we won't).

Signed-off-by: Geert Uytterhoeven <Geert.Uytterhoeven@sonycom.com>
Acked-by: Christoph Hellwig <hch@lst.de>
Signed-off-by: Jens Axboe <jens.axboe@oracle.com>