drivers: i2s: i2s_ll_stm32: Handle single clock source

Modify the function "i2s_stm32_set_clock" to handle the case where pclk_len
it is not greater then 1 and to ensure that the freq_in
variable is correctly set in both cases.

Signed-off-by: IBEN EL HADJ MESSAOUD Marwa <marwa.ibenelhadjmessaoud-ext@st.com>