[run] Fix up tx/rx frame size.

This specifically fixes that TX frames are large enough now to hold a 3900 odd
byte AMSDU (the little ones); me flipping it on earlier messed up transmit!

Tested:

• if_run, STA mode, TX/RX TCP/UDP iperf. TCP is now back to normal and correctly does ~ 3200 byte AMSDU/fast frames (2x1600ish byte MSDUs).