drivers/spi: stm32: Fix use of single dma channel

Current dma struture code didn't allowed only rx channel removal,
disabling tx channel (in spi client node) was leading compilation
issue.
Fix this by moving conditional code inside SPI_DMA_CHANNEL macro and
get the part of code which is present or removed (SPI_DMA_CHANNEL_INIT)
outside of {}.
Additionally, fix indentation on '\' in whole instance init macros

Signed-off-by: Erwan Gouriou <erwan.gouriou@linaro.org>