iommu/rockchip: Fix zap cache during device attach

rk_iommu_command() takes a struct rk_iommu and iterates over the slave
MMUs, so this is doubly wrong in that we're passing in the wrong pointer
and talking to MMUs that we shouldn't be.

Fixes: cd6438c5

 ("iommu/rockchip: Reconstruct to support multi slaves")
Cc: stable@vger.kernel.org
Signed-off-by: John Keeping <john@metanate.com>
Tested-by: Heiko Stuebner <heiko@sntech.de>
Reviewed-by: Heiko Stuebner <heiko@sntech.de>
Signed-off-by: Joerg Roedel <jroedel@suse.de>