net: xilinx: emacps: Fix receiving duplicate ping packets in 10mbps mode

The ping turnaround time measured for emacps in 10mbps mode is around 300ms.
So, increased the general purpose timeout value to 500ms for avoiding the rx
engine reset that occurs after every timer timeout and if there is no update
in the rx stat counters.

Signed-off-by: Punnaiah Choudary Kalluri <punnaia@xilinx.com>
Signed-off-by: Michal Simek <michal.simek@xilinx.com>