nvmem: hide unused nvmem_find_cell_by_index function

nvmem_find_cell_by_index() is only called from inside an #ifdef,
so we get a build warning without CONFIG_OF:

drivers/nvmem/core.c:496:1: error: 'nvmem_find_cell_by_index' defined but not used [-Werror=unused-function]

Move it into the same #ifdef as the caller to avoid the warning.

Fixes: e888d445

 ("nvmem: resolve cells from DT at registration time")
Signed-off-by: Arnd Bergmann <arnd@arndb.de>
Acked-by: Srinivas Kandagatla <srinivas.kandagatla@linaro.org>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>