mtd: nand/docg4: fix and improve read of factory bbt

This patch does two things related to reading the factory badblock table during
initialization: (1) fix error where a non-zero return code from
docg4_read_page() is assumed to be an error (it was later changed to be
max_bitflips; thanks to Brian Norris for bringing this to my attention a while
back), and (2) if there is an error reading the factory bbt, it tries reading
another (redundant) factory bbt table.

Signed-off-by: Mike Dunn <mikedunn@newsguy.com>
Acked-by: Robert Jarzmik <robert.jarzmik@free.fr>
Signed-off-by: Artem Bityutskiy <artem.bityutskiy@linux.intel.com>