[MTD] mtd_blkdevs.c: do not export 'mtd_blktrans_ops'

The mtd_blktrans_ops is not defined in any of the headers
and is indeed not used elsewhere in the kernel, so mark it
as static to reduce the warnings from sparse

drivers/mtd/mtd_blkdevs.c:204:32: warning: symbol 'mtd_blktrans_ops' was not declared. Should it be static?

Signed-off-by: Ben Dooks <ben-linux@fluff.org>
Signed-off-by: David Woodhouse <dwmw2@infradead.org>