The mhu_db_channel info is allocated per channel using devm_kzalloc from
mhu_db_mbox_xlate which gets called from mbox_request_channel. However
we are releasing the allocated mhu_db_channel info using plain kfree from
mhu_db_shutdown which is called from mbox_free_channel.

This leads to random crashes when the channel is freed like below one:

  Unable to handle kernel paging request at virtual address 0080000400000008
  [0080000400000008] address between user and kernel address ranges
  Internal error: Oops: 96000044 [#1] PREEMPT SMP
  Modules linked in: scmi_module(-)
  CPU: 1 PID: 2212 Comm: rmmod Not tainted 5.10.0-rc5 #31
  Hardware name: ARM LTD ARM Juno Development Platform/ARM Juno
  	Development Platform, BIOS EDK II Nov 19 2020
  pstate: 20000085 (nzCv daIf -PAN -UAO -TCO BTYPE=--)
  pc : release_nodes+0x74/0x230
  lr : devres_release_all+0x40/0x68
  Call trace:
   release_nodes+0x74/0x230
   devres_release_all+0x40/0x68
   device_release_driver_internal+0x12c/0x1f8
   driver_detach+0x58/0xe8
   bus_remove_driver+0x64/0xe0
   driver_unregister+0x38/0x68
   platform_driver_unregister+0x1c/0x28
   scmi_driver_exit+0x38/0x44 [scmi_module]
   __arm64_sys_delete_module+0x188/0x260
   el0_svc_common.constprop.0+0x80/0x1a8
   do_el0_svc+0x2c/0x98
   el0_sync_handler+0x160/0x168
   el0_sync+0x174/0x180
  Code: 1400000d eb07009f 54000460 f9400486 (f90004a6)
  ---[ end trace c55ffd306c140233 ]---

Fix it by replacing kfree with devm_kfree as required.

Fixes: 7002ca23 ("mailbox: arm_mhu: Add ARM MHU doorbell driver")
Reported-by: Cristian Marussi <cristian.marussi@arm.com>
Signed-off-by: Sudeep Holla <sudeep.holla@arm.com>
Tested-by: Cristian Marussi <cristian.marussi@arm.com>
Reviewed-by: Cristian Marussi <cristian.marussi@arm.com>
Signed-off-by: Jassi Brar <jaswinder.singh@linaro.org>