target/s390x/translate: Fix RNSBG instruction

target/s390x/translate: Fix RNSBG instruction

RNSBG is handled via the op_rosbg() helper function. But RNSBG has
the opcode 0xEC54, i.e. 0x54 as second byte, while op_rosbg() currently
checks for 0x55. This seems to be a typo, fix it to use 0x54 instead,
so that op_rosbg() does not abort() anymore if a program uses RNSBG.

I've checked with a simple test function that I now get the same results
with KVM and with TCG:

 static void test_rnsbg(void)
 {
	uint64_t r1, r2;

	r2 = 0xffff000000000000UL;
	r1 = 0x123456789bdfaaaaUL;
	asm volatile (" rnsbg %0,%1,12,61,16 " : "+r"(r1) : "r"(r2));

	printf("r1 afterwards: 0x%lx\n", r1);
 }

Buglink: https://bugs.launchpad.net/qemu/+bug/1860920
Signed-off-by: Thomas Huth <thuth@redhat.com>
Message-Id: <20200130133417.10531-1-thuth@redhat.com>
Fixes: d6c6372e186e ("target-s390: Implement R[NOX]SBG")
Reviewed-by: David Hildenbrand <david@redhat.com>
Signed-off-by: Cornelia Huck <cohuck@redhat.com>