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).