ipmi: Don't allow device module unload when in use

ipmi: Don't allow device module unload when in use

If something has the IPMI driver open, don't allow the device
module to be unloaded.  Before it would unload and the user would
get errors on use.

This change is made on user request, and it makes it consistent
with the I2C driver, which has the same behavior.

It does change things a little bit with respect to kernel users.
If the ACPI or IPMI watchdog (or any other kernel user) has
created a user, then the device module cannot be unloaded.  Before
it could be unloaded,

This does not affect hot-plug.  If the device goes away (it's on
something removable that is removed or is hot-removed via sysfs)
then it still behaves as it did before.

Reported-by: tony camuso <tcamuso@redhat.com>
Signed-off-by: Corey Minyard <cminyard@mvista.com>
Tested-by: tony camuso <tcamuso@redhat.com>