chore(uart): set LSE as source only if LSE is ready #823

Job	Run time
Check CMake usage (NUCLEO_F091RC)	49s
Check CMake usage (NUCLEO_F103RB)	43s
Check CMake usage (NUCLEO_F207ZG)	42s
Check CMake usage (NUCLEO_F303RE)	43s
Check CMake usage (NUCLEO_F411RE)	45s
Check CMake usage (NUCLEO_F767ZI)	43s
Check CMake usage (NUCLEO_G0B1RE)	45s
Check CMake usage (NUCLEO_G474RE)	47s
Check CMake usage (NUCLEO_H563ZI)	49s
Check CMake usage (NUCLEO_H743ZI2)	47s
Check CMake usage (NUCLEO_L073RZ)	43s
Check CMake usage (NUCLEO_L152RE)	42s
Check CMake usage (NUCLEO_L476RG)	42s
Check CMake usage (NUCLEO_L552ZE_Q)	44s
Check CMake usage (NUCLEO_U083RC)	34s
Check CMake usage (NUCLEO_U575ZI_Q)	41s
Check CMake usage (P_NUCLEO_WB55RG)	35s
Check CMake usage (NUCLEO_WBA55CG)	39s
Check CMake usage (NUCLEO_WL55JC1)	33s
	13m 26s

Provide feedback