UHD Python API: X310 ATR TX not working

[keukenrol]

Issue Description

When performing a transmit or burst transmit, the ATR GPIO function does no work.
For receiving, it does. When using ATR_0X, toggling only happens when the RX is active.
The LED's on the radio do work as well, so it seems only the GPIO ports do not react to it.
Tested on USRP X310.

Setup Details

Setup code:
` PIN_0_MASK = (1 << 0)
PIN_1_MASK = (1 << 1)
PIN_MASK = PIN_0_MASK | PIN_1_MASK

usrp.set_gpio_attr("FP0A", "CTRL",   PIN_MASK, 0xFFF)
usrp.set_gpio_attr("FP0A", "DDR",    PIN_MASK, 0xFFF)
usrp.set_gpio_attr("FP0A", "ATR_0X", 0x000, 0xFFF)
usrp.set_gpio_attr("FP0A", "ATR_RX", PIN_MASK, 0xFFF)
usrp.set_gpio_attr("FP0A", "ATR_TX", PIN_MASK, 0xFFF)
usrp.set_gpio_attr("FP0A", "ATR_XX", 0x000, 0xFFF)`

Expected Behavior

Whenever the transmit is active it should make the GPIO pin go high, just like the status LED.

Actual Behaviour

Nothing happens.

Steps to reproduce the problem

Setup the ATR for TX high in the Python api and run it.

Additional Information

This did not work with UHD 4.6 either. I've upgraded the FPGA firmware (HG version) but no difference was observed.