usb: cdc_acm: fix interrupt mode
usb: cdc_acm: fix interrupt mode

The CDC ACM driver has to emulate the TX and RX interrupts from the USB
ones. However it does not correctly emulate them as "TX buffer empty"
and "RX buffer not empty" interrupts.

For "TX buffer empty" interrupt:
- Reading the interrupt status should not clear the interrupt
- Enabling the interrupt should fire an interrupt if the TX buffer is
  empty
- An interrupt should be triggered when the USB device get configured

For "RX buffer not empty" interrupt:
- Reading the interrupt status should not clear the interrupt
- Enabling the interrupt should fire an interrupt if the RX buffer is
  not empty

This make the a console on the USB CDC ACM port usable for the shell
and the logs.

Signed-off-by: Aurelien Jarno <bceb368e7f2cb351af47298f32034f0587bbe4a6@aurel32.net>

nashif/zephyr,ldts/zephyr,Vudentz/zephyr,Vudentz/zephyr,Vudentz/zephyr,explora26/zephyr,finikorg/zephyr,punitvara/zephyr,punitvara/zephyr,nashif/zephyr,GiulianoFranchetto/zephyr,ldts/zephyr,nashif/zephyr,finikorg/zephyr,ldts/zephyr,Vudentz/zephyr,zephyrproject-rtos/zephyr,GiulianoFranchetto/zephyr,GiulianoFranchetto/zephyr,zephyrproject-rtos/zephyr,finikorg/zephyr,Vudentz/zephyr,explora26/zephyr,galak/zephyr,galak/zephyr,nashif/zephyr,GiulianoFranchetto/zephyr,finikorg/zephyr,galak/zephyr,GiulianoFranchetto/zephyr,punitvara/zephyr,explora26/zephyr,zephyrproject-rtos/zephyr,galak/zephyr,Vudentz/zephyr,finikorg/zephyr,ldts/zephyr,explora26/zephyr,punitvara/zephyr,nashif/zephyr,zephyrproject-rtos/zephyr,explora26/zephyr,ldts/zephyr,galak/zephyr,zephyrproject-rtos/zephyr,punitvara/zephyr