net: fix new sunrpc kernel-doc warning

Fix new kernel-doc warning in sunrpc:

Warning(net/sunrpc/xprt.c:196): No description found for parameter 'xprt'

Signed-off-by: Randy Dunlap <rdunlap@xenotime.net>
Signed-off-by: David S. Miller <davem@davemloft.net>