net: cadence_gem: Remove incorrect assert()

net: cadence_gem: Remove incorrect assert()

Don't assert on RX descriptor settings when the receiver is
disabled. This fixes an issue with incoming packets on an
unused GEM.

Reported-by: mbilal <muhammad_bilal@mentor.com>
Signed-off-by: Edgar E. Iglesias <edgar.iglesias@xilinx.com>
Reviewed-by: Philippe Mathieu-Daudé <philmd@redhat.com>
Message-id: 20181123135450.24829-2-edgar.iglesias@gmail.com
Reviewed-by: Peter Maydell <peter.maydell@linaro.org>
Signed-off-by: Peter Maydell <peter.maydell@linaro.org>