nvmem: mtk-efuse: use stack for nvmem_config instead of malloc'ing it

nvmem_register() copies all the members of nvmem_config to
nvmem_device.  So, nvmem_config is one-time use data during
probing.  There is no point to keep it until the driver detach.
Using stack should be no problem because nvmem_config is pretty
small.

Signed-off-by: Masahiro Yamada <yamada.masahiro@socionext.com>
Acked-by: Sean Wang <sean.wang@mediatek.com>
Signed-off-by: Srinivas Kandagatla <srinivas.kandagatla@linaro.org>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>