mtd: nand: omap: fix error return code in omap_nand_probe()

Fix to return a negative error code from the error handling case instead
of 0, to more closely match the rest of this function.

Signed-off-by: Wei Yongjun <yongjun_wei@trendmicro.com.cn>
Acked-by: Ezequiel Garcia <ezequiel.garcia@free-electrons.com>
Acked-by: Pekon Gupta <pekon@ti.com>
Signed-off-by: Brian Norris <computersforpeace@gmail.com>