BACKPORT: firmware: arm_ffa: Add missing remove callback to ffa_bus_type

Currently the arm_ffa firmware driver can be built as module and hence
all the users of FFA driver. If any driver on the ffa bus is removed or
unregistered, the remove callback on all the device bound to the driver
being removed should be callback. For that to happen, we must register
a remove callback on the ffa_bus which is currently missing. This results
in the probe getting called again without the previous remove callback
on a device which may result in kernel crash.

Fix the issue by registering the remove callback on the FFA bus.

Link: https://lore.kernel.org/r/20210924092859.3057562-1-sudeep.holla@arm.com


Fixes: e7818584 ("firmware: arm_ffa: Add initial FFA bus support for device enumeration")
Reported-by: Jens Wiklander <jens.wiklander@linaro.org>
Tested-by: Jens Wiklander <jens.wiklander@linaro.org>
Signed-off-by: Sudeep Holla <sudeep.holla@arm.com>
(cherry picked from commit 244f5d59)
(backported void->int return value for remove callback)
Change-Id: Ifc076ec9af4d60d5b7111dbc8ec4c2ad5b441b6f
Bug: 168585974
Signed-off-by: Will Deacon <willdeacon@google.com>