nvmem: Declare nvmem_cell_read() consistently

nvmem_cell_read() is declared as void * if CONFIG_NVMEM is enabled, and
as char * otherwise. This can result in a build warning if CONFIG_NVMEM
is not enabled and a caller asigns the result to a type other than char *
without using a typecast. Use a consistent declaration to avoid the
problem.

Fixes: e2a5402e ("nvmem: Add nvmem_device based consumer apis.")
Cc: Srinivas Kandagatla <srinivas.kandagatla@linaro.org>
Signed-off-by: Guenter Roeck <linux@roeck-us.net>
Signed-off-by: Srinivas Kandagatla <srinivas.kandagatla@linaro.org>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>