mips/ide: flush dcache also if icache does not snoop dcache

mips/ide: flush dcache also if icache does not snoop dcache

If this is not done then the new just read data which remains in dcache
will not make it into icache on time. Thus the CPU loads invalid data
and executes crap. The result is that the user is not able to execute
anything from its IDE based media while reading plain data is still
working well.

This problem has been reported as Debian #404951
http://bugs.debian.org/404951
http://comments.gmane.org/gmane.linux.ide/45092

Signed-off-by: Sebastian Andrzej Siewior <sebastian@breakpoint.cc>
Signed-off-by: John Crispin <blogic@openwrt.org>
Patchwork: http://patchwork.linux-mips.org/patch/2820/