usb/cdc_acm : Unsupported functions return error code

Poll functions were not implemented for the cdc_acm
USB uart since the underlying USB device controller
always uses interrupts.
We return -ENOTSUP and 0 rather than leaving unimplemented.

Jira: ZEP-595

Change-Id: I02d9e693694d03296da3c815cdda53f151e06f6d
Signed-off-by: Jithu Joseph <jithu.joseph@intel.com>