lantiq: ltq-tapi: add kernel 5.10 compatiblity

lantiq: ltq-tapi: add kernel 5.10 compatiblity

Due to SCHED_FIFO being a broken scheduler model, all users of
sched_setscheduler() are converted to sched_set_fifo_low() upstream and
sched_setscheduler() is no longer exported.

The callback handling of the tasklet API was redesigned and the macros
using the old syntax renamed to _OLD.

Signed-off-by: Mathias Kresin <dev@kresin.me>
(cherry picked from commit 31f3f797004ad318a1de88ec9cfdece523ee46d9)
[Add DECLARE_TASKLET handling for kernel 5.4.235 too]
Signed-off-by: Hauke Mehrtens <hauke@hauke-m.de>