staging: comedi: avoid releasing legacy minors automatically

`comedi_alloc_board_minor()` is called for both reserved "legacy"
devices created during module initialization, and for dynamically
created devices (via `comedi_auto_config()`).  All the minor device
numbers from 0 to `comedi_num_legacy_minors - 1` are for legacy devices
and all those from `comedi_num_legacy_minors` to
`COMEDI_NUM_BOARD_MINORS - 1` are for dynamically created devices.

`comedi_release_hardware_device()` is called (via
`comedi_auto_unconfig()`) when a dynamically created device is being
removed.  It needs to search the table of minor device numbers to see
which one is associated with the hardware device.  It currently starts
the search at minor device number 0.  Change it to start from
`comedi_num_legacy_minors` to skip over those belonging to legacy
devices.  Also change `comedi_alloc_board_minor()` to skip the legacy
devices when searching for a free minor device number for the
dynamically created device.  (The validity of the `hardware_device`
parameter is used to distinguish the legacy devices from the dynamically
created ones.)

Signed-off-by: Ian Abbott <abbotti@mev.co.uk>
Reviewed-by: H Hartley Sweeten <hsweeten@visionengravers.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>