mtd: Fix kernel NULL pointer dereference in physmap.c

During the probe for physmap platform flash devices there are a
number error exit conditions that all do a goto err_out which
then calls physmap_flash_remove().  In that function one of the
cleanup steps is:

#ifdef CONFIG_MTD_CONCAT
	if (info->cmtd != info->mtd[0])
		mtd_concat_destroy(info->cmtd);
#endif

This test will succeed since info->cmtd == NULL and info->mtd[0] is
valid.

Fix this by exiting the remove function when info->cmtd == NULL.

Also, cleanup the #ifdef CONFIG_MTD_PARTITIONS stuff by using
mtd_has_partitions().

Signed-off-by: H Hartley Sweeten <hsweeten@visionengravers.com>
Signed-off-by: David Woodhouse <David.Woodhouse@intel.com>