We don't need to check if the boot source is NAND in cases when we
already know for a fact that we booted from something else. To avoid
that, move the NAND check to be done inside of default branch of
the preceeding switch statement.

Signed-off-by: Andrey Smirnov <andrew.smirnov@gmail.com>
Signed-off-by: Sascha Hauer <s.hauer@pengutronix.de>