UBI: rename scan.c to attach.c

Finally, rename the scan.c file. Now adding fastmap support won't look that
hacky anymore.
Signed-off-by: Artem Bityutskiy <artem.bityutskiy@linux.intel.com>

drivers/mtd/ubi/Makefile

 obj-$(CONFIG_MTD_UBI) += ubi.o
 
-ubi-y += vtbl.o vmt.o upd.o build.o cdev.o kapi.o eba.o io.o wl.o scan.o
+ubi-y += vtbl.o vmt.o upd.o build.o cdev.o kapi.o eba.o io.o wl.o attach.o
 ubi-y += misc.o debug.o
 
 obj-$(CONFIG_MTD_UBI_GLUEBI) += gluebi.o