Fix address evaluates always as true warnings

This was an example of a compiler warning when the switch -Waddress was passed:

atmel_softpack_libraries/usb/device/core/USBD.c: In function 'USBD_RequestHandler':
atmel_softpack_libraries/usb/device/core/USBD.c:149:14: warning: the address of
    'USBDCallbacks_RequestReceived' will always evaluate as 'true' [-Waddress]
     else if (USBDCallbacks_RequestReceived) {

The test for existense of USBDCallbacks_RequestReceived is removed since there
is always a default implementation of these function.
1 file changed
tree: 6b92bf89e1c9a397ba7a06811f3a880b703df315
  1. minimal/
  2. sam3s_example/
  3. clk_calc.py