s390/irq: simplify do_softirq_own_stack()

do_softirq_own_stack() is always called from task context and
therefore it is not necessary to check if the async stack is
currently used.
Remove the check and directly switch to async stack.

Reviewed-by: Sven Schnelle <svens@linux.ibm.com>
Signed-off-by: Heiko Carstens <hca@linux.ibm.com>
Signed-off-by: Vasily Gorbik <gor@linux.ibm.com>