const: make block_device_operations const

arch/powerpc/sysdev/axonram.c

arch/um/drivers/ubd_kern.c

drivers/block/DAC960.c

drivers/block/amiflop.c

drivers/block/aoe/aoeblk.c

drivers/block/ataflop.c

drivers/block/brd.c

drivers/block/cciss.c

drivers/block/cpqarray.c

drivers/block/floppy.c

drivers/block/hd.c

drivers/block/loop.c

drivers/block/mg_disk.c

drivers/block/nbd.c

drivers/block/osdblk.c

drivers/block/paride/pcd.c

drivers/block/paride/pd.c

drivers/block/paride/pf.c

drivers/block/pktcdvd.c

drivers/block/ps3disk.c