nvmem: fix a 'makes pointer from integer without a cast' build warning

nvmem_register() returns a pointer, not a long int. Use ERR_CAST() to
cast the struct gpio_desc pointer to struct nvmem_device.

Reported-by: kbuild test robot <lkp@intel.com>
Fixes: 2a127da4

 ("nvmem: add support for the write-protect pin")
Signed-off-by: Bartosz Golaszewski <bgolaszewski@baylibre.com>
Acked-by: Srinivas Kandagatla <srinivas.kandagatla@linaro.org>