mtd: nand: don't walk past end of oobfree[]

Resolve issue noted by Sneha:  when computing oobavail from
the list of free areas in the OOB, don't assume there will
always be an unused slot at the end.  With ECC_HW_SYNDROME
and 4KiB page chips, it's fairly likely there *won't* be one.

Signed-off-by: David Brownell <dbrownell@users.sourceforge.net>
Cc: "Narnakaje, Snehaprabha" <nsnehaprabha@ti.com>"
Signed-off-by: Artem Bityutskiy <Artem.Bityutskiy@nokia.com>
Signed-off-by: David Woodhouse <David.Woodhouse@intel.com>