net: abort network initialization if the PHY driver fails

Now that phy_startup() can return an actual error code, check for that error
code and abort network initialization if the PHY fails.

Signed-off-by: Timur Tabi <timur@freescale.com>
Acked-by: Nobuhiro Iwamamatsu <nobuhiro.iwamatsu.yj@renesas.com> (sh_eth part)
Acked-by: Stephan Linz <linz@li-pro.net> (Xilinx part, xilinx_axi_emac and xilinx_ll_temac)
Reviewed-by: Marek Vasut <marex@denx.de> (FEC part)