net/e1000: add Tx/Rx tuning parameters

The optimal values of several transmission & reception related
parameters, such as burst sizes, descriptor ring sizes, and number
of queues, varies between different network interface devices. This
patch allows individual PMDs to specify preferred parameter values.

Signed-off-by: Remy Horton <remy.horton@intel.com>
Reviewed-by: Ferruh Yigit <ferruh.yigit@intel.com>