Allow specific interrupt level for DMA ISR.

Page 982.
Fixes https://forum.pjrc.com/threads/54711-Teensy-4-0-First-Beta-Test?p=196233&viewfull=1#post196233
- maybe this is needed for T3x, too? too late here to check this now.

fixes #266

I'm planning to merge the teensyduino 1.20 branch of DmaSpi to master tomorrow and then you can remove that hint, or update it please.

Changed  DMABaseClass::source() and destination() to use p's address instead of its value in the if-clause. Taking the value could result in unwated read operations from I/O locations, resulting in data loss.

Was set for 16 bit size.