Increase baseline SPI bulk transfer speeds #89
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This decreases the byte wise transaction from 26us to 19us, a 25% improvement. It does it by moving the iomTransfer struct to global and assigning the values that don't change into the begin function.
These changes along with the SDfat changes (incoming) make for some spectacular performance:
With SPI clock freq at 48MHz we get
write: 286 / 289 kb/s
read: 958 / 958 kb/s
On a clean format, 8GB Sandisk microSD card.
There was also a bug that allowed the core SPI freq to go above 48MHz. This PR fixes that as well.