mtd: Fix compile failure and error path in physmap.c

Commit 4b56ffca ("mtd: Fix kernel NULL
pointer dereference in physmap.c") introduced a couple of bugs.

It neglected to run the loop of map_destroy() calls in
physmap_flash_remove(), if !info->cmtd, which would happen if that
function was called to clean up errors during probe.

It also failed to compile if CONFIG_MTD_PARTITIONS was not defined.

Reported-By: Atsushi Nemoto <anemo@mba.ocn.ne.jp>
Signed-off-by: H Hartley Sweeten <hsweeten@visionengravers.com>
Signed-off-by: David Woodhouse <David.Woodhouse@intel.com>