spi(all): ensure halt is idempotent

[gen2thomas]

Solved issues and/or description of the change

see #1170

The fix is done in a generic way by adding unexposed wrapper functions in "spi_driver.go". Those functions check for connection is nil before calling the "connection.Func()". Additionally on "driver.Halt()" the connection is set to nil explicitly, so potential users of "d.beforeHalt" (normally functions are named "shutdown") can decide on this variable what to do.

Manual test

none

Checklist

• The PR's target branch is 'hybridgroup:dev'
• I have added tests that prove my fix is effective or that my feature works
• New and existing unit tests pass locally with my changes (e.g. by run make test_race)
• No linter errors exist locally (e.g. by run make fmt_check)
• I have performed a self-review of my own code