fmem: hotplug notifier should do nothing if fmem is inactive

If fmem discovers it has no memory to manage, it will
stay in an uninitialized state. However, this is
not discovered until long after the hotplug notifier
is set up (which must be in the fmem_init function).

Since fmem has not been initialized, the hotplug notifier
runs and incorrectly causes memory hotremove to fail, as
it bases its decision on uninitialized data.

The fix is for the hotplug notifier to just return
immediately if fmem is in the uninitialized state.

Change-Id: Ia647470d64fe1926be404af419f5f516e6b59c05
Signed-off-by: Larry Bassel <lbassel@codeaurora.org>