iommu/mediatek: Fix NULL pointer dereference when printing dev_name

When larbdev is NULL (in the case I hit, the node is incorrectly set
iommus = <&iommu NUM>), it will cause device_link_add() fail and
kernel crashes when we try to print dev_name(larbdev).

Let's fail the probe if a larbdev is NULL to avoid invalid inputs from
dts.

It should work for normal correct setting and avoid the crash caused
by my incorrect setting.

Error log:
[   18.189042][  T301] Unable to handle kernel NULL pointer dereference at virtual address 0000000000000050
...
[   18.344519][  T301] pstate: a0400005 (NzCv daif +PAN -UAO)
[   18.345213][  T301] pc : mtk_iommu_probe_device+0xf8/0x118 [mtk_iommu]
[   18.346050][  T301] lr : mtk_iommu_probe_device+0xd0/0x118 [mtk_iommu]
[   18.346884][  T301] sp : ffffffc00a5635e0
[   18.347392][  T301] x29: ffffffc00a5635e0 x28: ffffffd44a46c1d8
[   18.348156][  T301] x27: ffffff80c39a8000 x26: ffffffd44a80cc38
[   18.348917][  T301] x25: 0000000000000000 x24: ffffffd44a80cc38
[   18.349677][  T301] x23: ffffffd44e4da4c6 x22: ffffffd44a80cc38
[   18.350438][  T301] x21: ffffff80cecd1880 x20: 0000000000000000
[   18.351198][  T301] x19: ffffff80c439f010 x18: ffffffc00a50d0c0
[   18.351959][  T301] x17: ffffffffffffffff x16: 0000000000000004
[   18.352719][  T301] x15: 0000000000000004 x14: ffffffd44eb5d420
[   18.353480][  T301] x13: 0000000000000ad2 x12: 0000000000000003
[   18.354241][  T301] x11: 00000000fffffad2 x10: c0000000fffffad2
[   18.355003][  T301] x9 : a0d288d8d7142d00 x8 : a0d288d8d7142d00
[   18.355763][  T301] x7 : ffffffd44c2bc640 x6 : 0000000000000000
[   18.356524][  T301] x5 : 0000000000000080 x4 : 0000000000000001
[   18.357284][  T301] x3 : 0000000000000000 x2 : 0000000000000005
[   18.358045][  T301] x1 : 0000000000000000 x0 : 0000000000000000
[   18.360208][  T301] Hardware name: MT6873 (DT)
[   18.360771][  T301] Call trace:
[   18.361168][  T301]  dump_backtrace+0xf8/0x1f0
[   18.361737][  T301]  dump_stack_lvl+0xa8/0x11c
[   18.362305][  T301]  dump_stack+0x1c/0x2c
[   18.362816][  T301]  mrdump_common_die+0x184/0x40c [mrdump]
[   18.363575][  T301]  ipanic_die+0x24/0x38 [mrdump]
[   18.364230][  T301]  atomic_notifier_call_chain+0x128/0x2b8
[   18.364937][  T301]  die+0x16c/0x568
[   18.365394][  T301]  __do_kernel_fault+0x1e8/0x214
[   18.365402][  T301]  do_page_fault+0xb8/0x678
[   18.366934][  T301]  do_translation_fault+0x48/0x64
[   18.368645][  T301]  do_mem_abort+0x68/0x148
[   18.368652][  T301]  el1_abort+0x40/0x64
[   18.368660][  T301]  el1h_64_sync_handler+0x54/0x88
[   18.368668][  T301]  el1h_64_sync+0x68/0x6c
[   18.368673][  T301]  mtk_iommu_probe_device+0xf8/0x118 [mtk_iommu]
...

Cc: Robin Murphy <robin.murphy@arm.com>
Cc: Yong Wu <yong.wu@mediatek.com>
Reported-by: kernel test robot <lkp@intel.com>
Fixes: 635319a4 ("media: iommu/mediatek: Add device_link between the consumer and the larb devices")
Signed-off-by: Miles Chen <miles.chen@mediatek.com>
Reviewed-by: Yong Wu <yong.wu@mediatek.com>
Reviewed-by: AngeloGioacchino Del Regno <angelogioacchino.delregno@collabora.com>
Link: https://lore.kernel.org/r/20220505132731.21628-1-miles.chen@mediatek.com


Signed-off-by: Joerg Roedel <jroedel@suse.de>