mtd: nand: remove unnecessary extern from driver headers

'extern' is not necessary for function declarations.

scripts/checkpatch.pl with --strict option reports the following:

  CHECK: extern prototypes should be avoided in .h files

Signed-off-by: Masahiro Yamada <yamada.masahiro@socionext.com>
Signed-off-by: Boris Brezillon <boris.brezillon@free-electrons.com>