xen-netback: fix a BUG() during initialization

Commit edafc132 ("xen-netback: making the bandwidth limiter runtime settable")
introduced the capability to change the bandwidth rate limit at runtime.
But it also introduced a possible crashing bug.

If netback receives two XenbusStateConnected without getting the
hotplug-status watch firing in between, then it will try to register the
watches for the rate limiter again.  But this triggers a BUG() in the watch
registration code.

The fix modifies connect() to remove the possibly existing packet-rate
watches before trying to install those watches.  This behaviour is in line
with how connect() deals with the hotplug-status watch.

Signed-off-by: Imre Palik <imrep@amazon.de>
Cc: Matt Wilson <msw@amazon.com>
Acked-by: Wei Liu <wei.liu2@citrix.com>
Signed-off-by: David S. Miller <davem@davemloft.net>