stmhal: Protect SD card DMA transactions against USB MSC contention.
Consider the following scenario: SD card is being read by pyboard; USB irq comes in for MSC read request; SD card needs to be read from within USB irq while SD read is already ongoing. Such contention needs to be avoided. This patch provides a simple solution, to raise the irq priority above that of the USB irq during SD DMA transfers. Pyboard and PC can now read from the SD card at the same time (well, reads are interleaved).