mtd/nand/sh_flctl: Move function mtd_to_flctl to fix build failure

This patch fixes a build failure[1] by simply moving the function mtd_to_flctl
beneath the definition of sh_flctl which it uses.

BF introduced by patch
'mtd/nand/sh_flctl: Replace the dangerous mtd_to_flctl macro' (67026418

)

Signed-off-by: Peter Huewe <peterhuewe@gmx.de>
Signed-off-by: David Woodhouse <David.Woodhouse@intel.com>