comedi: comedi_test: fix check for valid scan_begin_src in waveform_ai_cmdtest()

comedi: comedi_test: fix check for valid scan_begin_src in waveform_ai_cmdtest()

Commit 783ddaebd397 ("staging: comedi: comedi_test: support
scan_begin_src == TRIG_FOLLOW") neglected to add a test that
`scan_begin_src` has only one bit set.  The allowed values are
`TRIG_FOLLOW` and `TRIG_TIMER`, but the code incorrectly also allows
`TRIG_FOLLOW | TRIG_TIMER`.  Add a call to
`comedi_check_trigger_is_unique()` to check that only one trigger source
bit is set.

Fixes: 783ddaebd397 ("staging: comedi: comedi_test: support scan_begin_src == TRIG_FOLLOW")
Cc: stable <stable@kernel.org>
Signed-off-by: Ian Abbott <abbotti@mev.co.uk>
Link: https://patch.msgid.link/20260422162138.36003-1-abbotti@mev.co.uk
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>