[MTD] Makefile fix for mtdsuper

We want drivers/mtd/{mtdcore, mtdsuper, mtdpart}.c to be built and linked
into the same mtd.ko module. Fix the Makefile to ensure this, and remove
duplicate MODULE_ declarations in mtdpart.c, as mtdcore.c already has them.

Signed-off-by: Satyam Sharma <satyam@infradead.org>
Signed-off-by: David Woodhouse <dwmw2@infradead.org>