nvmem: meson-efuse: simplify read callback

Most of the code and variables in the read callback is not necessary.
Keep only what is required.

Signed-off-by: Jerome Brunet <jbrunet@baylibre.com>
Signed-off-by: Srinivas Kandagatla <srinivas.kandagatla@linaro.org>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>