[SCSI] fusion: fix warning when not using procfs

Fixes the following warning:
drivers/message/fusion/mptbase.c:129: warning: 'mpt_proc_root_dir' defined but not used
also moves it from public data section since it is static.

Signed-off-by: Erik Ekman <erik@kryo.se>
Acked-by: "Desai, Kashyap" <Kashyap.Desai@lsi.com>
Signed-off-by: James Bottomley <James.Bottomley@suse.de>