staging: comedi: use refcount while reading /proc/comedi

In the seq_file 'show' handler for "/proc/comedi" - `comedi_read()` in
"comedi/proc.c",  call `comedi_dev_get_from_minor()` instead of
`comedi_dev_from_minor()` to increment the reference counter for the
`struct comedi_device` while it is being examined.  Call
`comedi_dev_put()` to decrement the reference afterwards.  Also acquire
the `attach_lock` rwsem while checking whether the device is attached.

Signed-off-by: Ian Abbott <abbotti@mev.co.uk>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>