spi: stm32-qspi: Trigger DMA only if more than 4 bytes to transfer

In order to optimize accesses to spi flashes, trigger a DMA only
if more than 4 bytes has to be transferred.

DMA transfer preparation's cost becomes negligible above 4 bytes to
transfer. Below this threshold, indirect transfer give more throughput.

mtd_speedtest shows that page write throughtput increases :
  - from 779 to 853 KiB/s (~9.5%) with s25fl512s SPI-NOR.
  - from 5283 to 5666 KiB/s (~7.25%) with Micron SPI-NAND.

Signed-off-by: Christophe Kerello <christophe.kerello@foss.st.com>
Signed-off-by: Patrice Chotard <patrice.chotard@foss.st.com>
Link: https://lore.kernel.org/r/20210419121541.11617-3-patrice.chotard@foss.st.com
Signed-off-by: Mark Brown <broonie@kernel.org>