sb16: fix interrupt acknowledgement

SoundBlaster 16 emulation is very broken and consumes a lot of CPU, but a
small fix was suggested offlist and it is enough to fix some games.  I
got Epic Pinball to work with the "SoundBlaster Clone" option.

The processing of the interrupt register is wrong due to two missing
"not"s.  This causes the interrupt flag to remain set even after the
Acknowledge ports have been read (0x0e and 0x0f).

The line was introduced by commit 85571bc7 (audio merge (malc), 2004-11-07),
but the code might have been broken before because I did not look closely
at the huge patches from 10 years ago.
Reported-by: NJoshua Bair <j_bair@bellsouth.net>
Cc: Gerd Hoffmann <kraxel@redhat.com>
Cc: qemu-stable@nongnu.org
Signed-off-by: NPaolo Bonzini <pbonzini@redhat.com>
Signed-off-by: NGerd Hoffmann <kraxel@redhat.com>

hw/audio/sb16.c

@@ -999,7 +999,7 @@ static IO_READ_PROTO (dsp_read)
         retval = (!s->out_data_len || s->highspeed) ? 0 : 0x80;
         if (s->mixer_regs[0x82] & 1) {
             ack = 1;
-            s->mixer_regs[0x82] &= 1;
+            s->mixer_regs[0x82] &= ~1;
             qemu_irq_lower (s->pic);
         }
         break;
@@ -1008,7 +1008,7 @@ static IO_READ_PROTO (dsp_read)
         retval = 0xff;
         if (s->mixer_regs[0x82] & 2) {
             ack = 1;
-            s->mixer_regs[0x82] &= 2;
+            s->mixer_regs[0x82] &= ~2;
             qemu_irq_lower (s->pic);
         }
         break;