staging: comedi: comedi_fops: absorb comedi_free_board_minor()

This function is only called by comedi_cleanup_board_minors() and the
'minor' parameter will always be < COMEDI_NUM_BOARD_MINORS.

For aesthetics, absorb the function and remove the unnecessary BUG_ON().

Split the comedi_clear_board_minor() out to clarify that the return value
is a comedi_device pointer.

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>