[bug] Spikeglx digital channel extraction harcodes to eight channels

[h-mayorquin]

We should fix this.

This calculation of channel index is also something that I had hard-coded and will work only when there are exactly 8 digital channels saved (which was the cases for our setup, but isn't generally true); but not when you save fewer channels or have I/O devices that have more than 8 digital channels.

Originally posted by @manimoh in https://github.com/NeuralEnsemble/python-neo/pull/1657#discussion_r2005742902