SUNRPC: Clean up the svc_xprt_flags() macro

Make this macro more conventional:
 - Use BIT() instead of open-coding " 1UL << "
 - Don't display the "XPT_" in every flag name

Reviewed-by: Jeff Layton <jlayton@kernel.org>
Signed-off-by: Chuck Lever <chuck.lever@oracle.com>