spi: stm32: fix stm32_spi_prepare_mbr() that halves spi clk for every run

When this driver is used with a driver that uses preallocated spi_transfer
structs. The speed_hz is halved by every run. This results in:

spi_stm32 44004000.spi: SPI transfer setup failed
ads7846 spi0.0: SPI transfer failed: -22

Example when running with DIV_ROUND_UP():
- First run; speed_hz = 1000000, spi->clk_rate 125000000
  div 125 -> mbrdiv = 7, cur_speed = 976562
- Second run; speed_hz = 976562
  div 128,00007 (roundup to 129) -> mbrdiv = 8, cur_speed = 488281
- Third run; speed_hz = 488281
  div 256,000131072067109 (roundup to 257) and then -EINVAL is returned.

Use DIV_ROUND_CLOSEST to allow to round down and allow us to keep the
set speed.

Signed-off-by: Sean Nyekjaer <sean@geanix.com>
Link: https://lore.kernel.org/r/20221103080043.3033414-1-sean@geanix.com
Signed-off-by: Mark Brown <broonie@kernel.org>