spi: stm32: fix stm32_spi_prepare_mbr in case of odd clk_rate

spi: stm32: fix stm32_spi_prepare_mbr in case of odd clk_rate

[ Upstream commit 9cc61973bf9385b19ff5dda4a2a7e265fcba85e4 ]

Fix spi->clk_rate when it is odd to the nearest lowest even value because
minimum SPI divider is 2.

Signed-off-by: Amelie Delaunay <amelie.delaunay@st.com>
Signed-off-by: Alain Volmat <alain.volmat@st.com>
Link: https://lore.kernel.org/r/1597043558-29668-4-git-send-email-alain.volmat@st.com
Signed-off-by: Mark Brown <broonie@kernel.org>
Signed-off-by: Sasha Levin <sashal@kernel.org>