fail operations if DMA bouncebuffer alloc fails
this fixes an issue with DMA to SPI6 on STM32H7. On the H7 SRAM4 can be used for DMA and BDMA to all devices except for SDMMC which uses IDMA. by separating out DMA of sdcard data from other data we can arrange for DMA to all devices to work
we need 512 bytes for microSD
this makes our bouncebuffers available for ChibiOS system drivers. This is needed for SDMMCv1 on STM32F7