Add device funtion for usb serial console, so we can open /dev/console
when we use a usb serial device as console.

(Typecast removed as noted by Sergei Shtylyov)
Signed-off-by: NKevin Hao <kexin.hao@windriver.com>
Signed-off-by: NAlan Cox <alan@redhat.com>
Signed-off-by: NLinus Torvalds <torvalds@linux-foundation.org>