Make ethrx_src array static, this is local to clk-stm32mp1.c

Fixes: e9ed1ef1 ("clk: stm32mp1: Add parent_data to ETHRX clock")
Reported-by: kernel test robot <lkp@intel.com>
Signed-off-by: Marek Vasut <marex@denx.de>
Link: https://lore.kernel.org/r/20221023164607.556118-1-marex@denx.deAcked-by: Gabriel Fernandez <gabriel.fernandez@foss.st.com>
Signed-off-by: Stephen Boyd <sboyd@kernel.org>