ARM: omap2: gpmc-onenand: Replace pr_err() with dev_err()

Do this becasue dev_err() is preferred over pr_err() and because
it will match gpmc-nand, thus the code shows looks more consistent.

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>