nvmem: properly handle returned value nvmem_reg_read

Function nvmem_reg_read can return a non zero value indicating an error.
This returned value must be read and error propagated to
nvmem_cell_prepare_write_buffer. Silence the following gcc warning (W=1):

drivers/nvmem/core.c:1093:9: warning: variable 'rc' set but
 not used [-Wunused-but-set-variable]

Signed-off-by: Mathieu Malaterre <malat@debian.org>
Signed-off-by: Srinivas Kandagatla <srinivas.kandagatla@linaro.org>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>