staging: comedi: rtd520: factor out the board reset

Factor the common code that does the low-level reset of the board
out of rtd_init_board() and rtc_detach().

Fix the if test in rtd_detach() before doing the reset, devpriv->lcfg
could be NULL at this point.

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>