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]