The algorithm to detect serial mode can be shared between i.M6 and
i.MX7 as wall as simplified a bit by replacing swich goto/break/return
termination logic with more trivial if statements.

This commit also sets the stage for additional improvements in the
commits that follow.

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