MFD: mcp-core: sanitize host creation/removal

host_unregister() gives us no chance between removing the device
and the mcp data structure being freed to access the data inbetween,
which drivers may need to do if they need to iounmap() pointers in
their private data structures.

Therefore, re-jig the interfaces, which are now, on creation:

	mcp = mcp_host_alloc()
	if (mcp) {
		ret = mcp_host_add(mcp, data);

		if (!ret)
			mcp_host_free(mcp);
	}

and on removal:

	mcp_host_del(mcp);
	... access mcp ...
	mcp_host_free(mcp);

The free does the final put_device() on the struct device as one would
expect.

Acked-by: Jochen Friedrich <jochen@scram.de>
Signed-off-by: Russell King <rmk+kernel@arm.linux.org.uk>