spi: stm32: perform small transfer in polling mode

spi: stm32: perform small transfer in polling mode

In case of interrupt based transfer, when the transfer is very
small, relying on interrupts leads to lower performances than if
the transfer were done using polling on the registers.

Add a module parameter "polling_limit_us" to indicate the threshold
in us from which a transfer would be done polling the registers rather
than relying on interrupts.

Signed-off-by: Alain Volmat <alain.volmat@foss.st.com>
Link: https://patch.msgid.link/20251218-stm32-spi-enhancements-v2-3-3b69901ca9fe@foss.st.com
Signed-off-by: Mark Brown <broonie@kernel.org>