[S390] qdio: inline qdio_perf_stat_inc

Move qdio_perf_stat_inc to the header file so it can be inlined.
Remove unused qdio_perf_stat_dec.

Signed-off-by: Jan Glauber <jang@linux.vnet.ibm.com>
Signed-off-by: Martin Schwidefsky <schwidefsky@de.ibm.com>