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.