fx2lafw: Blink LED on pin PA1 during acquisition.

All FX2 based logic analyzer devices will now toggle PA1 during
acquisition, and keep it high while idle.

On fx2grok-tiny and fx2grok-flat there's a LED connected to PA1.
Thus, those devices will visually indicate whether a firmware has been
uploaded (LED will light up and stay like that while the device is
idle). During any acquisition the LED will blink.

The PA1 pin is not used on most other FX2 based LA devices, so that
shouldn't cause any issues (tested on hardware).

On certain devices with LA and analog support the PA1 pin is used for
other purposes (e.g. Hantek 6022BL, Instrustar ISDS205X) but even there
this won't cause any issues in practice (tested on hardware).