net: gem: Do not initialize BDs again

BDs can be correctly setup just once and init function
performs only phy autodetection and enabling RX/TX.
RX/TX are disabled in halt function.

This patch solves the problem with repeatable tftp transfers.

Signed-off-by: Michal Simek <michal.simek@xilinx.com>
Reviewed-by: Tom Rini <trini@ti.com>