mtd: nand: davinci: simplify error handling

There is not needed to use a lot of names for err handling.
It complicates code support and reading.

Acked-by: Santosh Shilimkar <santosh.shilimkar@ti.com>
Reviewed-by: Grygorii Strashko <grygorii.strashko@ti.com>
Reviewed-by: Taras Kondratiuk <taras@ti.com>
Signed-off-by: Ivan Khoronzhuk <ivan.khoronzhuk@ti.com>
Signed-off-by: Brian Norris <computersforpeace@gmail.com>