In non blocking mode, the read function is expected
to return weather data is available for reading or not.
In case data are available but the size does not match
the expected size, the function read should return whatever
data is available.
Previously, the function was returning -1 with errno set
to EWOULDBLOCK even if the size of data in the buffer was
less than the requested size. It would only return the
available data if the size in the buffer was greater or equal
to the requested size.
The implementation of cdcacm_read is modified to return the avilable
data from the buffer even is the size is lesser than the requested
size.
