usb: device_next: Reduce code size on Full-Speed only configuration

Allow compiler optimizations to remove High-Speed handling code. Knowing
that maximum operating speed is Full-Speed allows to reduce bulk buffers
from 512 to 64 bytes. More RAM optimizations are possible but this
commit only gets the low hanging fruits.

Signed-off-by: Tomasz Moń <tomasz.mon@nordicsemi.no>