char: xillybus: Don't return -EFAULT on user-triggered flush

The API allows the application to flush a host-to-FPGA stream by calling
write() with the data count set to zero. Before this patch, copy_from_user()
was called with a non-zero byte count, which possibly made it attempt to read
from unmapped user memory. Such attempts caused the driver to return -EFAULT
instead of 0, even though the desired operation went through fine.

This patch ensures the driver returns 0 on a successful flush.

Signed-off-by: Eli Billauer <eli.billauer@gmail.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>