r8169: Disable clk during suspend / resume

Disable the clk during suspend to save power. Note that tp->clk may be
NULL, the clk core functions handle this without problems.

Reviewed-by: Andy Shevchenko <andriy.shevchenko@linux.intel.com>
Tested-by: Carlo Caione <carlo@endlessm.com>
Signed-off-by: Hans de Goede <hdegoede@redhat.com>
Signed-off-by: David S. Miller <davem@davemloft.net>