firmware: arm_ffa: Set handle field to zero in memory descriptor

As described in the commit 111a833d ("firmware: arm_ffa: Set
reserved/MBZ fields to zero in the memory descriptors") some fields in
the memory descriptor have to be zeroed explicitly. The handle field is
one of these, but it was left out from that change, fix this now.

Fixes: 111a833d

 ("firmware: arm_ffa: Set reserved/MBZ fields to zero in the memory descriptors")
Reported-by: Imre Kis <imre.kis@arm.com>
Signed-off-by: Balint Dobszay <balint.dobszay@arm.com>
Link: https://lore.kernel.org/r/20230601140749.93812-1-balint.dobszay@arm.com
Signed-off-by: Sudeep Holla <sudeep.holla@arm.com>