mtd: nand: onfi don't WARN if we are in 16 bits mode

Commit ff3206b2 ('mtd: nand: onfi need 
to be probed in 8 bits mode') adds a WARN if the onfi probe is in 16 
bits mode. This allows to detect driver that need to be fixed, but this 
is a bit noisy¹. Transform the WARN in a pr_err.

¹ http://article.gmane.org/gmane.linux.ports.arm.omap/91317



Signed-off-by: Matthieu CASTET <matthieu.castet@parrot.com>
Signed-off-by: David Woodhouse <David.Woodhouse@intel.com>