drivers: ram: stm32: fix compilation issue

If CONFIG_CLK flag is not set, compilation raises the
following error message:

drivers/ram/stm32_sdram.c: In function 'stm32_fmc_probe':
drivers/ram/stm32_sdram.c:154:2: error: 'ret' undeclared (first use in this function)
  ret = stm32_sdram_init(dev);

Signed-off-by: Patrice Chotard <patrice.chotard@st.com>
cc: Vikas Manocha <vikas.manocha@st.com>