clk: stm32mp1: remove error for disabled clock in stm32mp1_clk_get_parent

To disabled a clock in clock tree initialization for a mux of STM32MP15,
the selected clock source index is set with the latest possible index for
the number of bit used. Today this valid configuration cause a error
in U-Boot messages, for example with CLK_ETH_DISABLED, when this clock
is not needed for the used ETH PHY without crystal:

   no parents defined for clk id 123

This patch change the level of this message to avoid this trace for
valid clock tree.

Signed-off-by: Patrick Delaunay <patrick.delaunay@foss.st.com>
Reviewed-by: Patrice Chotard <patrice.chotard@foss.st.com>