driver core: add local subsys_get and subsys_put functions

We need to control the reference count of the subsys private structure
instead of directly manipulating the kset reference count of it, so wrap
that logic up in a subsys_get() and subsys_put() function to make it more
obvious as to what is happening.

Cc: "Rafael J. Wysocki" <rafael@kernel.org>
Link: https://lore.kernel.org/r/20230208111330.439504-2-gregkh@linuxfoundation.org
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>