chip->bits_per_cell which is used to determine the NAND cell type
(SLC/MLC) should always have a value != 0.
Complain loudly if the value is 0 in nand_is_slc() to catch use before
correct initialization.
Signed-off-by: NLothar Waßmann <LW@KARO-electronics.de>
Signed-off-by: NBoris Brezillon <boris.brezillon@free-electrons.com>