fix(st-clock): correct stm32_clk_parse_fdt_by_name

The fdt_getprop() function sets the length to -1 if the property is not
found. We should then not use it later in stm32_clk_parse_fdt_by_name()
in that case. Directly set *nb to 0U and return 0 if the property is not
found.

Signed-off-by: Yann Gautier <yann.gautier@foss.st.com>
Change-Id: I19c5c953f392cdc768e0b1f3f240fc99a73a049c