drm/bridge: tc358775: make standby GPIO optional

The stby pin is optional. It is only needed for power-up and down
sequencing. It is not needed, if the power rails cannot by dynamically
enabled.

Because the GPIO is now optional, remove the error message.

Signed-off-by: Michael Walle <mwalle@kernel.org>
Reviewed-by: Dmitry Baryshkov <dmitry.baryshkov@linaro.org>
Acked-by: Krzysztof Kozlowski <krzysztof.kozlowski@linaro.org>
Signed-off-by: Tony Lindgren <tony@atomide.com>
Reviewed-by: Robert Foss <rfoss@kernel.org>
Signed-off-by: Robert Foss <rfoss@kernel.org>
Link: https://patchwork.freedesktop.org/patch/msgid/20240225062008.33191-6-tony@atomide.com