Fix silent stall of next68k esp(4) SCSI.

next68k esp(4) driver requires nextdma(4) interrupts at ipl 6
during ncr53c9x_intr() for esp(4) at ipl 3.  It worked before
netbsd-6, but on netbsd-5 splbio() was changed from ipl 3 to 6
for SMP support and on netbsd-6 ncr53c9x driver was changed to
use mutex(9) so nextdma interrupts were no longer allowed during
ncr53c9x interrupt hander.

For now, just call mutex_exit(9) and mutex_enter(9) during
wating nextdma(4) interrupts in esp_dma_intr() handler.
This could be wrong and the interrupt hander for nextdma should
be reorganized, but it just works.

sys/arch/next68k/dev/esp.c

@@ -509,6 +509,7 @@ esp_dma_intr(struct ncr53c9x_softc *sc)
 			}
 #endif
 
+			mutex_exit(&sc->sc_lock);	/* for nextdma intr */
 			while (!nextdma_finished(nsc)) {
 			/* esp_dma_isactive(sc)) { */
 				NDTRACEIF (ndtrace_addc('w'));
@@ -602,7 +603,7 @@ esp_dma_intr(struct ncr53c9x_softc *sc)
 
 			}
 		out:
-			;
+			mutex_enter(&sc->sc_lock);	/* for nextdma intr */
 
 #ifdef ESP_DEBUG
 /* 			esp_dma_nest--; */