hw/omap_gpmc: Modify correct field when writing IRQSTATUS register

Writing to IRQSTATUS should affect irqst, not irqen -- error
spotted by Andrzej Zaborowski.

Signed-off-by: Peter Maydell <peter.maydell@linaro.org>
Signed-off-by: Andrzej Zaborowski <andrew.zaborowski@intel.com>
diff --git a/hw/omap_gpmc.c b/hw/omap_gpmc.c
index e27b93c..7fc82a2 100644
--- a/hw/omap_gpmc.c
+++ b/hw/omap_gpmc.c
@@ -639,7 +639,7 @@
         break;
 
     case 0x018:	/* GPMC_IRQSTATUS */
-        s->irqen &= ~value;
+        s->irqst &= ~value;
         omap_gpmc_int_update(s);
         break;