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>

drivers/char/xillybus/xillybus_core.c

@@ -1237,6 +1237,8 @@ static ssize_t xillybus_write(struct file *filp, const char __user *userbuf,
 					unsigned char *tail;
 					int i;
 
+					howmany = 0;
+
 					end_offset_plus1 = bufpos >>
 						channel->log2_element_size;