cdc_acm : Restrict writing more than 4 bytes into TX USB Endpoint.

Sometime it is observed on the Arduino 101 that when we write more than
4 bytes into TX USB Endpoint, first 4 bytes are getting repeated
(frequency of occurrence ~1/3000).

This patch does following :-

1. In sample application "cdc_acm", it adds capability to
handle partial transfer data incase data is transferred partially
if exceeds maximum data transfer size.
2. It restricts write of more than  4 bytes into TX USB Endpoint.
This is work around to  avoid issue occarance.

Jira: ZEP-2074

Signed-off-by: Youvedeep Singh <youvedeep.singh@intel.com>
Signed-off-by: Andy Ross <andrew.j.ross@intel.com>
Signed-off-by: Anas Nashif <anas.nashif@intel.com>