spi: stm32: fix missing pointer assignment in case of dma chaining

spi: stm32: fix missing pointer assignment in case of dma chaining

Commit c4f2c05ab029 ("spi: stm32: fix pointer-to-pointer variables usage")
introduced a regression since dma descriptors generated as part of the
stm32_spi_prepare_rx_dma_mdma_chaining function are not well propagated
to the caller function, leading to mdma-dma chaining being no more
functional.

Fixes: c4f2c05ab029 ("spi: stm32: fix pointer-to-pointer variables usage")
Signed-off-by: Alain Volmat <alain.volmat@foss.st.com>
Acked-by: Antonio Quartulli <antonio@mandelbit.com>
Link: https://patch.msgid.link/20260224-spi-stm32-chaining-fix-v1-1-5da7a4851b66@foss.st.com
Signed-off-by: Mark Brown <broonie@kernel.org>