Fix Tx performance on urtwn(4) RTL8192EU devices.

Fix Tx performance on urtwn(4) RTL8192EU devices.

The "driver rate" bit which tells firmware to use the driver-provided
Tx rate is in a different spot of Tx descriptor double word 3.
Fix the corresponding macro definition.

The device was sending all frames at CCK1 because of this bug, regardless
of the Tx rate the driver had chosen.

tcpbench before:
Conn:   1 Mbps:        0.808 Peak Mbps:        1.108 Avg Mbps:        0.808

tcpbench with fix:
Conn:   1 Mbps:       21.078 Peak Mbps:       21.078 Avg Mbps:       21.078

ok jmatthew@