nand: omap2: Replace pr_err with dev_err

Usage of pr_err is frowned upon, so replace it with dev_err.

Acked-by: Roger Quadros <rogerq@ti.com>
Signed-off-by: Ezequiel Garcia <ezequiel@vanguardiasur.com.ar>
Signed-off-by: Brian Norris <computersforpeace@gmail.com>