samples: webusb: fix type of bytes_read

bytes_read should be an int, all uses of it in this sample are of type
int.  Lets clean this up so when building with newlib or in the future
when uint32_t might change its definition we don't have issues.

Change-Id: Ief7bfb48948df50cec45b4707ef7676d13173f34
Signed-off-by: Kumar Gala <kumar.gala@linaro.org>