mtd-utils: Fix wrong argument to sizeof in nanddump

Some temporary buffers are allocated with "sizeof(pointer) * count" as
size argument, which cannot possibly be correct.

Assuming what was meant was "sizeof(pointer[0]) * count" makes sense
in the context of how the buffers are used, but is actually pretty
pointless, since the buffers are unsigend char.

Signed-off-by: David Oberhollenzer <david.oberhollenzer@sigma-star.at>