iommu/arm-smmu-v3: Remove some unneeded init in arm_smmu_cmdq_issue_cmdlist()

iommu/arm-smmu-v3: Remove some unneeded init in arm_smmu_cmdq_issue_cmdlist()

Members of struct "llq" will be zero-inited, apart from member max_n_shift.
But we write llq.val straight after the init, so it was pointless to zero
init those other members. As such, separately init member max_n_shift
only.

In addition, struct "head" is initialised to "llq" only so that member
max_n_shift is set. But that member is never referenced for "head", so
remove any init there.

Removing these initializations is seen as a small performance optimisation,
as this code is (very) hot path.

Signed-off-by: John Garry <john.garry@huawei.com>
Link: https://lore.kernel.org/r/1624293394-202509-1-git-send-email-john.garry@huawei.com
Signed-off-by: Will Deacon <will@kernel.org>