mtd: cleanup nand.h

- *var instead of * var
- proper multiline comment
- func(args) instead of func (args)
- 80 lines

So from
|total: 2 errors, 37 warnings, 654 lines checked
we got to one warning.

Signed-off-by: Sebastian Andrzej Siewior <bigeasy@linutronix.de>
Signed-off-by: Artem Bityutskiy <Artem.Bityutskiy@nokia.com>
Signed-off-by: David Woodhouse <David.Woodhouse@intel.com>