hw/nvram: Avoid unnecessary Xilinx eFuse backstore write

hw/nvram: Avoid unnecessary Xilinx eFuse backstore write

Add a check in the bit-set operation to write the backstore
only if the affected bit is 0 before.

With this in place, there will be no need for callers to
do the checking in order to avoid unnecessary writes.

Signed-off-by: Tong Ho <tong.ho@amd.com>
Reviewed-by: Alistair Francis <alistair.francis@wdc.com>
Reviewed-by: Francisco Iglesias <frasse.iglesias@gmail.com>
Reviewed-by: Philippe Mathieu-Daudé <philmd@linaro.org>
Signed-off-by: Peter Maydell <peter.maydell@linaro.org>