lantiq: ltq-tapi: fix compatibility with Linux 4.15+

lantiq: ltq-tapi: fix compatibility with Linux 4.15+

Linux 4.15 removes the init_timer() API. It's replaced by two functions:
- timer_setup() is used instead of init_timer() and also replaces the
  timer "function" (callback) setup.
- from_timer() has to be used to obtain the use-case specific data from
  a struct timer_list, which is now passed to the timer callback.

Update the timer API to be compatible with Linux 4.15+ so it compiles
with the upcoming Linux 4.19 kernel update.

Signed-off-by: Martin Blumenstingl <martin.blumenstingl@googlemail.com>