drivers/at25xxx: return read/written bytes to match MTD API [backport 2020.04]

[benpicco]

Backport of #13886

Contribution description

The MTD API expects read() to return the number of read bytes and write() to return the number of written bytes.
This also matches the behavior of periph/eeprom.h.

So change the driver API to match that.

Testing procedure

tests/driver_at25xxx should still work.

2020-04-17 11:58:34,252 # AT25XXX EEPROM driver test application
2020-04-17 11:58:34,252 # 
2020-04-17 11:58:34,254 # EEPROM size: 128 kiB
2020-04-17 11:58:34,257 # EEPROM page size: 256 bytes
2020-04-17 11:58:34,259 # EEPROM address length: 24 bits
2020-04-17 11:58:34,259 # 
2020-04-17 11:58:34,302 # ...
2020-04-17 11:58:34,303 # OK (3 tests)

Issues/PRs references

needed for #13877

[benpicco]

The driver was introduced during this release cycle, so the change of the API should also be part of this release.

[maribu]

I lost track of time. We are still in soft feature freeze, right? If so, we should get this in soon.

[benpicco]

It's hard feature freeze already, but I hoped I could still slip this API change it - since the driver was just added during this cycle, it would make it as if there was no API change at all 😉

[leandrolanzieri]

I'm fine with having this in the release. The API has not been released yet

[leandrolanzieri]

As in the original PR, we trust the output provided by @benpicco. ACK