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.

https://groups.google.com/a/arduino.cc/d/msg/developers/ls3hkviFYM4/XWT2LbfzNcgJ

Was set for 16 bit size.

http://forum.pjrc.com/threads/26064-Large-transfers-over-serial-on-Teensy-3-1
http://forum.arduino.cc/index.php/topic,165574.0.html

This patch simply checks if NUM_ENDPOINTS is defined, and will bypass the
built-in USB descriptor, device and memory code, and allows a custom setting.
This bug does not affect the IDE unless someone adds the custom option to
the boards.txt file, and provides replacement code in a library or sketch.
It can also be used to turn off USB by providing no replacements, while
allowing speeds >= 20MHz.

Switched order of parameters as suggested by PaulStoffregen.