USB: f_ccid: Fix the return value of the bulk read

The read should return the number bytes copied to the user space
buffer.  The current code is returning the value passed to the
read() system call.

We exactly know how many bytes host will send over the bulk out
endpoint before issuing the read.  Hence there are no issues
with the current code.  This issue is found during a unit test
which sends random number of bytes over the bulk out endpoint.

Change-Id: I52d28d53f0550f6937ae765f981168e31d6e5694
Signed-off-by: Pavankumar Kondeti <pkondeti@codeaurora.org>