Stellaris: Implement preliminary 'writen' member in LPC driver

Signed-off-by: Alexandru Gagniuc <mr.nuke.me@gmail.com>
Stellaris: Do not use a libopencm3 callback for OUT packets

Signed-off-by: Alexandru Gagniuc <mr.nuke.me@gmail.com>
Stellaris: Do not enable Tx or Rx USB interrupts

Only the control endpoint is interrupt-driven, while the bulk endpoints are
polled. There's no need to enable interrupts for other endpoints, so keep
them disabled.

Signed-off-by: Alexandru Gagniuc <mr.nuke.me@gmail.com>