net: zynq_gem: Fix reading of max-speed property

max-speed property is part of phynode and it has to be
read using ofnode_read_u32_default(). This fixes the issue
of incorrect max-speed read from DT.

Signed-off-by: Siva Durga Prasad Paladugu <siva.durga.paladugu@xilinx.com>
Acked-by: Joe Hershberger <joe.hershberger@ni.com>
Signed-off-by: Michal Simek <michal.simek@xilinx.com>