bsg: mark struct file_operations const

struct file_operations is generally const (to avoid false sharing and get compile time errors on accidental writing to this shared structure); bsg recently added one of these without the const keyword. Patch below marks it const....

Signed-off-by: Arjan van de Ven <arjan@linux.intel.com>
Signed-off-by: Jens Axboe <jens.axboe@oracle.com>