spi/mips-lantiq: make use of spi_finalize_current_message

Rather than calling m->complete() directly we choose the sane way and call
spi_finalize_current_message instead.

Signed-off-by: Thomas Langer <thomas.langer@lantiq.com>
Signed-off-by: John Crispin <blogic@openwrt.org>
Signed-off-by: Grant Likely <grant.likely@secretlab.ca>