clk: vc5: Abort clock configuration without upstream clock

In case the upstream clock are not set, which can happen in case the
VC5 has no valid upstream clock, the $src variable is used uninited
by regmap_update_bits(). Check for this condition and return -EINVAL
in such case.

Note that in case the VC5 has no valid upstream clock, the VC5 can
not operate correctly. That is a hardware property of the VC5. The
internal oscilator present in some VC5 models is also considered
upstream clock.

Signed-off-by: Marek Vasut <marek.vasut+renesas@gmail.com>
Cc: Alexey Firago <alexey_firago@mentor.com>
Cc: Laurent Pinchart <laurent.pinchart@ideasonboard.com>
Cc: Stephen Boyd <sboyd@kernel.org>
Cc: linux-renesas-soc@vger.kernel.org
[sboyd@kernel.org: Added comment about probe preventing this from
happening in the first place]
Signed-off-by: Stephen Boyd <sboyd@kernel.org>