mtd/maps/physmap: catch failure to register MTD_PHYSMAP_COMPAT device

If the default Kconfig values are used with MTD_PHYSMAP_COMPAT you end
up with a resource where end < start.  This causes __request_resource to
return a conflict which then returns an -EBUSY error code.  The current
physmap.c code just assumes that the platfom_device_register will always
succeed.

Catch this failure during the physmap_init and propogate the error.

Signed-off-by: H Hartley Sweeten <hsweeten@visionengravers.com>
Reported-by: Randy Dunlap <randy.dunlap@oracle.com>
Signed-off-by: David Woodhouse <David.Woodhouse@intel.com>