kernel: qca-ssdk: backport support for building as kernel module

kernel: qca-ssdk: backport support for building as kernel module

Currently, SSDK is rather special in the sense that its not being built as
a proper out of tree module at all but rather like a userspace application
and that involves a lot of make magic which unfortunately broke with make
version 4.4 and newer.

Luckily QCA finally added a way to build SSDK as an out of tree module
and it uses the kernel buildsystem which makes it compile with make 4.4
as well.
So lets backport the support for it and switch to using it.

Signed-off-by: Robert Marko <robimarko@gmail.com>