staging: comedi: usbduxfast: remove TRIG_TIMER from scan_begin_src

Currently the (*do_cmdtest) indicates that TRIG_TIMER is a valid scan_begin_src
but later this source is tested as -EINVAL. To simplify the code a bit just
remove the TRIG_TIMER source.

Signed-off-by: H Hartley Sweeten <hsweeten@visionengravers.com>
Cc: Ian Abbott <abbotti@mev.co.uk>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>