With nvmem framework, the reference counter of the nvmem device is
increased when we ask the device from the kernel via helper functions
like "nvmem_device_find()", "nvmem_device_get()", etc. If reference
count is zero, the driver can be unloaded (rmmod) during ongoing
operations.

Thus, the reference count should be incremented during add, and
decremented during delete.

Reported-by: Federico Vaga <federico.vaga@cern.ch>
Signed-off-by: Vaibhav Gupta <vaibhav.gupta@cern.ch>