net: zynq_gem: Initialize phyreg variable

In case of phyread()/phy_setup_op() timeout code is working with
uninitialized phyreg variable. Initialize this variable to make sure
that code it not working with random value.

Signed-off-by: Michal Simek <michal.simek@xilinx.com>
Acked-by: Joe Hershberger <joe.hershberger@ni.com>