zynq: gem: Increase the Rx buffer descriptors to 32

zynq: gem: Increase the Rx buffer descriptors to 32

Increase the Rx Buffer descriptors to 32. This will avoid
Rx buffer descriptors overflow if more packets were received
at one shot before we process the received ones.
This fixes the issue of intermittent timeouts during tftp
on a 1Gb connection with tftp server running on windows.

Signed-off-by: Siva Durga Prasad Paladugu <sivadur@xilinx.com>
Signed-off-by: Michal Simek <michal.simek@xilinx.com>