nvmem: core: Fix device reference leak

__nvmem_device_get() make use of bus_find_device() to get the relevant
device and this function increase the reference count of the device
found, however this is not accounted for anywhere. Fix
__nvmem_device_get() and __nvmem_device_put() to properly release this
reference count.

Signed-off-by: Alban Bedel <albeu@free.fr>
Signed-off-by: Srinivas Kandagatla <srinivas.kandagatla@linaro.org>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>