e1000e: Remove duplicate assignment of default rx/tx ring size

tx_ring/rx_ring size is assigned in function e1000_alloc_queues(), which is
called by e1000_sw_init() in the early stage of e1000_probe().

This patch just remove the duplicate assignment of this default ring size
value.

Signed-off-by: Wei Yang <weiyang@linux.vnet.ibm.com>
Reviewed-by: Gavin Shan <shangw@linux.vnet.ibm.com>
Reviewed-by: Da Yu Qiu <qiudayu@cn.ibm.com>
Tested-by: Aaron Brown <aaron.f.brown@intel.com>
Acked-by: Bruce Allan <bruce.w.allan@intel.com>
Signed-off-by: Jeff Kirsher <jeffrey.t.kirsher@intel.com>