firmware: arm_ffa: Move comment before the field it is documenting

This is consistent with the other comments in the struct.

Co-developed-by: Andrew Walbran <qwandor@google.com>
Signed-off-by: Andrew Walbran <qwandor@google.com>
Signed-off-by: Quentin Perret <qperret@google.com>
Reviewed-by: Sudeep Holla <sudeep.holla@arm.com>
Link: https://lore.kernel.org/r/20221116170335.2341003-3-qperret@google.com


Signed-off-by: Will Deacon <will@kernel.org>