Add 'no MRPC' version of switchtec_open function

[xyan264]

This new function opens a device without issuing MRPC command, so the contents in gas MRPC region is intact after the call.

[lsgunth]

Why?

[xyan264]

We've seen requests to fix the 'gas read/write' command issues like PR #259 .

The issue is caused by switchtec_open sending MRPC command during initialization, overwriting contents in MRPC GAS region (0x0-0x1000).

This new switchtec_open_no_mrpc function does not issue MRPC command, so it can be a good workaround for this issue, if needed.

[lsgunth]

People should simple not do that. In fact, I think the proper thing to do is to disallow access to the MRPC region of the gas entirely.

There's no way to know if you're stepping on someone's toes if you try to do MRPC through the GAS and there are much better ways to do it. (See the examples folder). So just say no.

[maquefel]

This pull request solves some issues with gas read/write - at least i could get temperature (i replaced every switchtec_open with switchtec_open_no_mrcp). But using modified tool with ChipLink hanged both host with PAX 42100 and slave connected to it. So @lsgunth is right it us a really dangerous thing.

On the other hand is it supposed that configuration should be done only by serial or ethernet terminal (that still uses gas read/write but in other form) ? This is quite dissapointing.