staging: comedi: hwdrv_apci3501: refactor apci3501_read_insn_timer()

The handling of the ADDIDATA_WATCHDOG and ADDIDATA_TIMER is identical.
Refactor this function to use a common code path for both timer modes.

Remove the incorrect dev_err() noise. The subdevice is valid but the
timer_mode is not supported.

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>