staging: comedi: drivers (core): remove BUG_ON in comedi_auto_unconfig()

comedi_find_board_minor() will always return a ninor number between
0 and < COMEDI_NUM_BOARD_MINORS, or -ENODEV if a minor is not found.

Remove the unnecessary BUG_ON().

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>