mtd: stm32: use parameter instead of default value

To set wait state, a hard coded value is used instead of using
latency parameter. stm32_flash_latency_cfg() is currently used
in arch/arm/mach-stm32/stm32f4/clock.c and in
drivers/clk/clk_stm32f7.c with, in both case, "5" as parameter.

Signed-off-by: Patrice Chotard <patrice.chotard@st.com>