[MTD] MAPS: esb2rom: use hotplug safe interfaces

Fairly self explanatory.  Keep a reference initially, drop it when we free up
the driver resources.

Signed-off-by: Alan Cox <alan@redhat.com>
Signed-off-by: Andrew Morton <akpm@osdl.org>
Signed-off-by: David Woodhouse <dwmw2@infradead.org>