Fix compiler warnings about ignored 'const' in EEPROM.h

[bxparks]

Fix for Issue #36

[matthijskooijman]

Looks good to me, thanks!

For future reference, it would have been fine to just create the PR, without also creating a separate issue (especially since the fix is so simple). No worries now, though :-)

[per1234]

Fixes arduino/Arduino#3783

[facchinm]

LGTM! Does it solve the issue for both 4.9 (AVR core 1.6.19) and 5.4 (AVR core 1.6.20) compilers?

[bxparks]

@facchinm: Did you mean to ask me that question?

I've only ever used Arduino IDE 1.8.5 (maybe 1.8.4?). As far as I can tell from package_index.json, all 3 of the most recent AVR core versions (1.6.19, 1.6.20, 1.6.21) use the same compiler:

            "toolsDependencies": [
              {
                "packager": "arduino",
              "paname": "avr-gcc",
             "naversion": "4.9.2-atmel3.5.4-arduino2"
              },

We have to go back to 1.6.17 to get to an AVR Core that uses a different compiler (4.9.2-atmel3.5.3-arduino2), and I verified that the bug is there as well and is fixed by this PR.

[per1234]

@bxparks Arduino IDE 1.8.6's bundled Arduino AVR Boards 1.6.22 uses avr-gcc '5.4.0-atmel3.6.1-arduino2. Arduino AVR Boards 1.6.22 hasn't been released separately from the IDE so that's why you don't see it in https://downloads.arduino.cc/packages/package_index.json. Perhaps facchinm got the version number mixed up.

[bxparks]

Ah, it was released a few days ago.
I installed IDE 1.8.6 and verified my PR fixes the warning in avr-gcc 5.4.0-atmel3.6.1-arduino2 (core 1.6.22) as well.

[facchinm]

Yes, I meant 1.6.22, sorry for the confusion.