iommu/mediatek-v1: Clean up redundant fwspec checks

iommu/mediatek-v1: Clean up redundant fwspec checks

The driver explicitly clears any existing fwspec before calling
mtk_iommu_v1_create_mapping(), but even if it didn't, the checks it's
doing there duplicate what iommu_fwspec_init() would do anyway. Clean
them up.

Reported-by: kernel test robot <lkp@intel.com>
Closes: https://lore.kernel.org/oe-kbuild-all/202407020415.KKnhPTUj-lkp@intel.com/
Signed-off-by: Robin Murphy <robin.murphy@arm.com>
Link: https://lore.kernel.org/r/7d6ebec135483f889af00eb376aa31c012efc3b2.1719919669.git.robin.murphy@arm.com
Signed-off-by: Will Deacon <will@kernel.org>