mtd/nand/sh_flctl: Replace the dangerous mtd_to_flctl macro

The original macro worked only when applied to variables named 'mtd'.
While this could have been fixed by simply renaming the macro argument,
a more type-safe replacement is preferred.

Signed-off-by: Ferenc Wagner <wferi@niif.hu>
Signed-off-by: Artem Bityutskiy <Artem.Bityutskiy@nokia.com>
Signed-off-by: David Woodhouse <David.Woodhouse@intel.com>