ooblen is read in nand_do_read_ops, despite never having been assigned a
value. Because ooblen is used to indicate how many bytes of oobbuf are
usable and oobbuf is NULL, use 0 as initial value.

Found on ARCH=sandbox using clang-analyzer.

Signed-off-by: Ahmad Fatoum <a.fatoum@pengutronix.de>
Signed-off-by: Sascha Hauer <s.hauer@pengutronix.de>