nvmem: fix nvmem_cell_read() return type doc

nvmem_cell_read() returns void *, not char *. This is a cleanup that got
left out of commit a6c50912

 ("nvmem: Declare nvmem_cell_read()
consistently").

Signed-off-by: Brian Norris <briannorris@chromium.org>
Fixes: a6c50912

 ("nvmem: Declare nvmem_cell_read() consistently")
Signed-off-by: Srinivas Kandagatla <srinivas.kandagatla@linaro.org>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>