MIPS: DEC: Export `ioasic_ssr_lock' to modules

Fix a modular `declance' regression caused by LMO commit bb46bf30d13f
("DECstation SCSI driver clean-ups.")

Signed-off-by: Maciej W. Rozycki <macro@linux-mips.org>
Cc: linux-mips@linux-mips.org
Patchwork: https://patchwork.linux-mips.org/patch/13256/
Signed-off-by: Ralf Baechle <ralf@linux-mips.org>