parisc: sba: Fix compile warning wrt list of SBA devices

[ Upstream commit eb3255ee ]

Fix this makecheck warning:
drivers/parisc/sba_iommu.c:98:19: warning: symbol 'sba_list'
	was not declared. Should it be static?

Signed-off-by: Helge Deller <deller@gmx.de>
Signed-off-by: Sasha Levin <sashal@kernel.org>