bus: moxtet: declare moxtet_bus_type as static

This symbol is not meant to be used from elsewhere.

Cc: Ben Dooks <ben.dooks@codethink.co.uk>
Cc: Christoph Hellwig <hch@infradead.org>
Cc: Gregory CLEMENT <gregory.clement@bootlin.com>
Signed-off-by: Marek Behún <marek.behun@nic.cz>
Signed-off-by: Gregory CLEMENT <gregory.clement@bootlin.com>