Staging: comedi: cleanup: remove unneeded null checks

These checks are obviously pointless because kfree() can handle null
dereferences.

But really the main problem is that if the pointers were null that would
cause problems on the ealier lines.  The dereferences would cause an
oops and the _release() functions use ->priv to determine which IRQ to
free.  I looked into it and quite a few of the detach functions assume
link->priv is non-null.  It seems like we can remove these checks.

Signed-off-by: Dan Carpenter <error27@gmail.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@suse.de>