net/9p/trans_fd.c: Fix unsigned return type

The function has an unsigned return type, but returns a negative constant
to indicate an error condition.  The result of calling the function is
always stored in a variable of type (signed) int, and thus unsigned can be
dropped from the return type.

A sematic match that finds this problem is as follows:
(http://coccinelle.lip6.fr/)

// <smpl>
@exists@
identifier f;
constant C;
@@

 unsigned f(...)
 { <+...
*  return -C;
 ...+> }
// </smpl>
Signed-off-by: NJulia Lawall <julia@diku.dk>
Signed-off-by: NDavid S. Miller <davem@davemloft.net>

net/9p/trans_fd.c

@@ -222,7 +222,7 @@ static void p9_conn_cancel(struct p9_conn *m, int err)
 	}
 }
 
-static unsigned int
+static int
 p9_fd_poll(struct p9_client *client, struct poll_table_struct *pt)
 {
 	int ret, n;