mtd: physmap: fix NULL pointer dereference in error path

commit e480814f ("[MTD] [MAPS] physmap:
fix wrong free and del_mtd_{partition,device}") introduces a NULL pointer
dereference in physmap_flash_remove when called from the error path in
physmap_flash_probe (if map_probe failed).

Call del_mtd_{partition,device} only if info->cmtd was not NULL.

Reported-by: pHilipp Zabel <philipp.zabel@gmail.com>
Signed-off-by: Atsushi Nemoto <anemo@mba.ocn.ne.jp>
Cc: David Woodhouse <dwmw2@infradead.org>
Cc: "Rafael J. Wysocki" <rjw@sisk.pl>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>