net: zynq: Don't overwrite gem_rclk_ctrl with default value

The gem[0-1]_rclk_ctrl registers control the source of the rx clock,
control and data signals and configure via ps7_init function. Don't
overwrite the register with the default value.

Signed-off-by: Stefan Herbrechtsmeier <stefan.herbrechtsmeier@weidmueller.com>
Reviewed-by: Joe Hershberger <joe.hershberger@ni.com>
Signed-off-by: Michal Simek <michal.simek@xilinx.com>