staging/fwserial: Annotate rcu pointers with __rcu

Fixes these sparse warnings:
drivers/staging/fwserial/fwserial.c:430:16: sparse: incompatible types in comparison expression (different address spaces)
drivers/staging/fwserial/fwserial.c:699:30: sparse: incompatible types in comparison expression (different address spaces)
drivers/staging/fwserial/fwserial.c:802:16: sparse: incompatible types in comparison expression (different address spaces)
drivers/staging/fwserial/fwserial.c:898:16: sparse: incompatible types in comparison expression (different address spaces)
drivers/staging/fwserial/fwserial.c:1842:14: sparse: incompatible types in comparison expression (different address spaces)

Reported-by: kbuild test robot <fengguang.wu@intel.com>
Signed-off-by: Peter Hurley <peter@hurleysoftware.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>

drivers/staging/fwserial/fwserial.h

@@ -280,7 +280,7 @@ struct fwtty_port {
 				   loopback:1;
 	unsigned long		   flags;
 
-	struct fwtty_peer	   *peer;
+	struct fwtty_peer __rcu	   *peer;
 
 	struct async_icount	   icount;
 	struct stats		   stats;