mtd: nand: Get rid of needless 'goto'

Using "goto" and "switch" statement only makes it harder to follow
control flow and doesn't bring any advantages. Rewrite the code to avoid
using "goto".

Signed-off-by: Brian Norris <computersforpeace@gmail.com>
Signed-off-by: Andrey Smirnov <andrew.smirnov@gmail.com>
Signed-off-by: Boris Brezillon <boris.brezillon@free-electrons.com>