hw/arm/pxa2xx_gpio: Fix handling of GPSR/GPCR reads

The PXA2xx GPIO GPSR and GPCR registers are write-only, with reads being
undefined behaviour. Instead of having GPCR return 31337 and GPSR return
the value last written, make both log the guest error and return 0.
Signed-off-by: NPeter Maydell <peter.maydell@linaro.org>
Reviewed-by: NPeter Crosthwaite <peter.crosthwaite@xilinx.com>

hw/arm/pxa2xx_gpio.c

@@ -36,7 +36,6 @@ struct PXA2xxGPIOInfo {
     uint32_t rising[PXA2XX_GPIO_BANKS];
     uint32_t falling[PXA2XX_GPIO_BANKS];
     uint32_t status[PXA2XX_GPIO_BANKS];
-    uint32_t gpsr[PXA2XX_GPIO_BANKS];
     uint32_t gafr[PXA2XX_GPIO_BANKS * 2];
 
     uint32_t prev_level[PXA2XX_GPIO_BANKS];
@@ -162,14 +161,14 @@ static uint64_t pxa2xx_gpio_read(void *opaque, hwaddr offset,
         return s->dir[bank];
 
     case GPSR:		/* GPIO Pin-Output Set registers */
-        printf("%s: Read from a write-only register " REG_FMT "\n",
-                        __FUNCTION__, offset);
-        return s->gpsr[bank];	/* Return last written value.  */
+        qemu_log_mask(LOG_GUEST_ERROR,
+                      "pxa2xx GPIO: read from write only register GPSR\n");
+        return 0;
 
     case GPCR:		/* GPIO Pin-Output Clear registers */
-        printf("%s: Read from a write-only register " REG_FMT "\n",
-                        __FUNCTION__, offset);
-        return 31337;		/* Specified as unpredictable in the docs.  */
+        qemu_log_mask(LOG_GUEST_ERROR,
+                      "pxa2xx GPIO: read from write only register GPCR\n");
+        return 0;
 
     case GRER:		/* GPIO Rising-Edge Detect Enable registers */
         return s->rising[bank];
@@ -217,7 +216,6 @@ static void pxa2xx_gpio_write(void *opaque, hwaddr offset,
     case GPSR:		/* GPIO Pin-Output Set registers */
         s->olevel[bank] |= value;
         pxa2xx_gpio_handler_update(s);
-        s->gpsr[bank] = value;
         break;
 
     case GPCR:		/* GPIO Pin-Output Clear registers */