[MTD] map_rom has NULL erase pointer

Which means if inftl or similar are loaded with it (which is a dumb thing
to do admittedly) it may oops.

Closes #8108

[dwmw2: change error to -EROFS to match write-protected flash]
Signed-off-by: Alan Cox <alan@redhat.com>
Signed-off-by: David Woodhouse <David.Woodhouse@intel.com>