net/cadence_gem: Don't rx packets when no rx buffer available

Return false from can_receive() when no valid buffer descriptor is
available. Ensures against mass packet droppage in some applications.

Signed-off-by: Peter Crosthwaite <peter.crosthwaite@xilinx.com>
Message-id: cde00ef774e84e2586bf10fd37b542f75bf36cfb.1386136219.git.peter.crosthwaite@xilinx.com
Signed-off-by: Peter Maydell <peter.maydell@linaro.org>
