ipu_idmac: fix spinlock type

fix a probably accidently dropped reference operator while calling
spin_unlock_restore to an ipu lock.

Signed-off-by: Luotao Fu <l.fu@pengutronix.de>
Cc: Guennadi Liakhovetski <lg@denx.de>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Dan Williams <dan.j.williams@intel.com>