Fix lwIP PPP glue

[kjbracey]

Description

EMAC changes broke PPP glue - fix it up. In particular, PPP steals
the netif->state pointer, so we can't use it as a back pointer to
our LWIP::Interface.

Pull request type

[X] Fix
[ ] Refactor
[ ] New target
[ ] Feature
[ ] Breaking change

[0xc0170]

/morph build

[mbed-ci]

Build : SUCCESS

Build number : 2173
Build artifacts/logs : http://mbed-os.s3-website-eu-west-1.amazonaws.com/?prefix=builds/7030/

Triggering tests

/morph test
/morph uvisor-test
/morph export-build
/morph mbed2-build

[mbed-ci]

Test : SUCCESS

Build number : 1961
Test logs :http://mbed-os-logs.s3-website-us-west-1.amazonaws.com/?prefix=logs/7030/1961

[mbed-ci]

Exporter Build : FAILURE

Build number : 1797
Build artifacts/logs : http://mbed-os.s3-website-eu-west-1.amazonaws.com/?prefix=builds/exporter/7030/

[kjbracey]

Exporter failed on CPU stats examples - as if the patches done to make #7009 pass hadn't been applied?

[kjbracey]

Tip of this branch precedes #7009. Is the exporter testing the branch tip rather than the merge result?

That would explain it - it's seeing DEVICE_LOWPOWERTIMER on the tip here when the test expects DEVICE_LPTICKER.

[kjbracey]

Ah, 7009 was not merged when exporter test was launched. Will pass next time.

/morph export-build

[mbed-ci]

Exporter Build : SUCCESS

Build number : 1798
Build artifacts/logs : http://mbed-os.s3-website-eu-west-1.amazonaws.com/?prefix=builds/exporter/7030/