mtd: nand: refactor print messages

Add a nice "nand:" prefix to all pr_xxx() messages. This allows
to get rid of the "NAND" words in messages, given the context
is already given by the prefix.

Remove the __func__ report from messages where it's not needed and refactor
the device detection messages to show itself in several lines.

Signed-off-by: Ezequiel Garcia <ezequiel.garcia@free-electrons.com>
Signed-off-by: Brian Norris <computersforpeace@gmail.com>