iommu/arm-smmu-v3: Consolidate identical timeouts

We have separate (identical) timeout values for polling for a queue to
drain and waiting for an MSI to signal CMD_SYNC completion. In reality,
we only wait for the command queue to drain if we're waiting on a sync,
so just merged these two timeouts into a single constant.

Signed-off-by: Will Deacon <will.deacon@arm.com>