spi: stm32: Fix logical error in stm32_spi_prepare_mbr()

stm32_spi_prepare_mbr() is returning an error value when div is less
than SPI_MBR_DIV_MIN *and* greater than SPI_MBR_DIV_MAX, which always
evaluates to false. This should change to use *or*.

Signed-off-by: Christos Gkekas <chris.gekas@gmail.com>
Signed-off-by: Mark Brown <broonie@kernel.org>