nvmem: core: constify nvmem_cell_read_variable_common() return value

nvmem: core: constify nvmem_cell_read_variable_common() return value

The caller doesn't modify the memory pointed to by the pointer so it
can be const.

Suggested-by: Stephen Boyd <swboyd@chromium.org>
Signed-off-by: Douglas Anderson <dianders@chromium.org>
Reviewed-by: Stephen Boyd <swboyd@chromium.org>
Signed-off-by: Srinivas Kandagatla <srinivas.kandagatla@linaro.org>
Link: https://lore.kernel.org/r/20210611083348.20170-9-srinivas.kandagatla@linaro.org
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>