UBI: fix use of "VID" vs. "EC" in header self-check

Looks like a typo, using UBI_EC_HDR_SIZE_CRC (note the "EC") to compute
the CRC for the VID header.

This shouldn't cause any functional change, as both structures are 64
bytes. Verified with:

	BUILD_BUG_ON(UBI_VID_HDR_SIZE_CRC != UBI_EC_HDR_SIZE_CRC);

Reported here:
http://lists.infradead.org/pipermail/linux-mtd/2013-September/048570.html

Reported by: Bill Pringlemeir <bpringlemeir@gmail.com>
Signed-off-by: Brian Norris <computersforpeace@gmail.com>
Signed-off-by: Richard Weinberger <richard@nod.at>