mtd: lpddr: drop unnecessary zeroing

We allocate the "mtd" structure using kzalloc which means we do not have
to initialize unused MTD function pointers to NULL, since it is safe to
assume in Linux that NULL contains all zeroes.

Signed-off-by: Artem Bityutskiy <artem.bityutskiy@linux.intel.com>
Signed-off-by: David Woodhouse <David.Woodhouse@intel.com>