qat: Move CWARNFLAGS down so it works.

CWARNFLAGS must be added to after including kern.mk, otherwise all the
warnings set in kern.mk are ignored. This is why the
-Wmissing-prototypes warnings were not seen previously.

While here, drop -Wno-pointer-sign as it is doesn't seem to be needed,
but add -Wno-cast-qual to silence warnings for various casts of const
pointers to non-const pointers.

Reviewed by: MichalX.Gulbicki_intel.com, markj
Sponsored by: DARPA
Differential Revision: https://reviews.freebsd.org/D36332