net: ipv6-pe: Verify that hmac calculation was ok

Make sure to check return values of mbedtls hmac APIs so that
the digest is calculated properly.

Fixes #75259
Fixes #75260
Fixes #75261

Coverity-CID: 366271
Coverity-CID: 366277
Coverify-CID: 366279

Signed-off-by: Jukka Rissanen <jukka.rissanen@nordicsemi.no>