[MIPS] DEC: Initialise ioasic_ssr_lock

Fix the definition of the ioasic_ssr_lock spinlock to include a proper 
initialisation.

Signed-off-by: Maciej W. Rozycki <macro@linux-mips.org>
Signed-off-by: Ralf Baechle <ralf@linux-mips.org>