usb: cdc_acm: Allow to create several CDC ACM devices
usb: cdc_acm: Allow to create several CDC ACM devices

Change global device pointer to device list where every device
registers.

Signed-off-by: Andrei Emeltchenko <a6565233ddc88e4fb9c66c1d70743223493f2ed4@intel.com>

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