firmware: arm_scmi: Return a literal instead of a variable

In this context "return scmi_dev;" and "return NULL;" are equivalent
but it is more readable to return a literal.

Signed-off-by: Dan Carpenter <error27@gmail.com>
Reviewed-by: Cristian Marussi <cristian.marussi@arm.com>
Link: https://lore.kernel.org/r/Y/Yx8pOdf8rNhPVe@kili


Signed-off-by: Sudeep Holla <sudeep.holla@arm.com>