drivers/block/mg_disk.c: use resource_size()

Use resource_size() for ioremap.

The ioremap appears to be passing the incorrect size for the platform
resource.  Unfortunately, I can't locate a user in mainline to verify
this.  Using resource_size should be the correct fix.

Signed-off-by: H Hartley Sweeten <hsweeten@visionengravers.com>
Acked-by: unsik Kim <donari75@gmail.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Jens Axboe <jens.axboe@oracle.com>