staging: comedi: amplc_pci224: use comedi_fc helpers to validate timer args

Use the comedi_fc helper cfc_check_trigger_arg_is() to validate the
scan_begin_arg when the scan_begin_src is TRIG_TIMER.

For aesthetics reasons, rename the local variable 'tmp' to 'arg'.

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