mei: bus: Reset event_cb when disabling a device

After cancelling all reads from the disable hook, we need to reset the
event_cb pointer as well or else we won't be able to set a new one up
when re-enabling the device.

Acked-by: Tomas Winkler <tomas.winkler@intel.com>
Signed-off-by: Samuel Ortiz <sameo@linux.intel.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>