cadence_gem: Avoid infinite loops with a misconfigured buffer

A guest can write zero to the DMACFG resulting in an infinite loop when
it reaches the while(bytes_to_copy) loop.

To avoid this issue enforce a minimum size for the RX buffer. Hardware
does not have this enforcement and relies on the guest to set a non-zero
value.

Signed-off-by: Alistair Francis <alistair.francis@xilinx.com>
Reported-by: Li Qiang <liqiang6-s@360.cn>
Reported-by: P J P <ppandit@redhat.com>
Message-id: 84bb1c391b833275da3f573d4972920cea34c188.1466539342.git.alistair.francis@xilinx.com
Signed-off-by: Peter Maydell <peter.maydell@linaro.org>
