ARM: omap2: gpmc-onenand: Replace printk KERN_ERR with dev_warn()

Since the condition is not an error but a warning, replace
printk KERN_ERR with dev_warn.

Signed-off-by: Ezequiel Garcia <ezequiel.garcia@free-electrons.com>
Reviewed-by: Jon Hunter <jon-hunter@ti.com>
Signed-off-by: Jon Hunter <jon-hunter@ti.com>