staging: comedi: comedi_fops: remove BUG_ON() in comedi_dev_get_from_board_minor()

This function is only called by comedi_dev_get_from_minor() and the 'minor'
value will always be < COMEDI_NUM_BOARD_MINORS. Remove the unnecessary
BUG_ON().

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>