nvmem: core: fix nvmem_cell_write inline function

nvmem_cell_write's buf argument uses different types based on
the configuration of CONFIG_NVMEM. The function prototype for
enabled NVMEM uses 'void *' type, but the static dummy function
for disabled NVMEM uses 'const char *' instead. Fix the different
behaviour by always expecting a 'void *' typed buf argument.

Fixes: 7a78a7f7 ("power: reset: nvmem-reboot-mode: use NVMEM as reboot mode write interface")
Reported-by: kbuild test robot <lkp@intel.com>
Cc: Han Nandor <nandor.han@vaisala.com>
Cc: Srinivas Kandagatla <srinivas.kandagatla@linaro.org>
Cc: linux-kernel@vger.kernel.org
Signed-off-by: Sebastian Reichel <sebastian.reichel@collabora.com>
Reviewed-By: Han Nandor <nandor.han@vaisala.com>
Signed-off-by: Srinivas Kandagatla <srinivas.kandagatla@linaro.org>