tile-srom: allow the driver to be built as a module

The code was already configured that way, but the Kconfig
file didn't support requesting it.

A buglet caused a null pointer deref when unloading the
module, but this commit also corrects that issue.

Signed-off-by: Chris Metcalf <cmetcalf@mellanox.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>