e1000e: 82574/82583 performance improvement

Increasing the transmit fifo by 4K (by decreasing the receive fifo size
specified in .pba by the same amount) increases Tx performance.

Signed-off-by: Bruce Allan <bruce.w.allan@intel.com>
Tested-by: Emil Tantilov <emil.s.tantilov@intel.com>
Signed-off-by: Jeff Kirsher <jeffrey.t.kirsher@intel.com>