firmware: arm_scmi: fix of_node leak in scmi_mailbox_check

firmware: arm_scmi: fix of_node leak in scmi_mailbox_check

[ Upstream commit fa7fe29a645b4da08efe8ff2392898b88f9ded9f ]

of_parse_phandle_with_args() requires the caller to call of_node_put() on
the returned args->np pointer. Otherwise the reference count will remain
incremented.

However, in this case, since we don't actually use the returned pointer,
we can simply pass in NULL.

Fixes: aa4f886f3893f ("firmware: arm_scmi: add basic driver infrastructure for SCMI")
Signed-off-by: Steven Price <steven.price@arm.com>
Signed-off-by: Sudeep Holla <sudeep.holla@arm.com>
Signed-off-by: Sasha Levin <sashal@kernel.org>