arm_mpam: Fix a NULL pointer dereference on unbinding after an error interrupt

arm_mpam: Fix a NULL pointer dereference on unbinding after an error interrupt

If a user unbinds an MSC after mpam_disable() has been run in response
to an error interrupt then a dereference of a NULL pointer occurs as
mpam_disable() sets the drvdata to NULL. Add an early return to the driver
remove callback to avoid this.

Fixes: f04046f2577a ("arm_mpam: Add probe/remove for mpam msc driver and kbuild boiler plate")
Signed-off-by: Ben Horgan <ben.horgan@arm.com>
Signed-off-by: Will Deacon <will@kernel.org>