fel: Handle error and don't exit if MMU is not enabled.

On allwinner SOCs A33,H3, A83T MMU is not enabled by BROM, so don't exit
if MMU is not enabled by BROM and return NULL.
It was reported on A33 tablet and Orange Pi H3 Board.
I've tested it on A33 tablet and A83T Dev Board by allwinner.
Signed-off-by: VishnuPatekar <vishnupatekar0510@gmail.com>
Acked-by: Siarhei Siamashka <siarhei.siamashka@gmail.com>

fel.c

@@ -502,7 +502,7 @@ uint32_t aw_get_sctlr(libusb_device_handle *usb)
 
 uint32_t *aw_backup_and_disable_mmu(libusb_device_handle *usb)
 {
-	uint32_t *tt = malloc(16 * 1024);
+	uint32_t *tt = NULL;
 	uint32_t ttbr0 = aw_get_ttbr0(usb);
 	uint32_t sctlr = aw_get_sctlr(usb);
 	uint32_t i;
@@ -519,8 +519,8 @@ uint32_t *aw_backup_and_disable_mmu(libusb_device_handle *usb)
 	};
 
 	if (!(sctlr & 1)) {
-		fprintf(stderr, "MMU is not enabled by BROM\n");
-		exit(1);
+		pr_info("MMU is not enabled by BROM\n");
+		return NULL;
 	}
 
 	if ((sctlr >> 28) & 1) {
@@ -533,6 +533,7 @@ uint32_t *aw_backup_and_disable_mmu(libusb_device_handle *usb)
 		exit(1);
 	}
 
+	tt = malloc(16 * 1024);
 	pr_info("Reading the MMU translation table from 0x%08X\n", ttbr0);
 	aw_fel_read(usb, ttbr0, tt, 16 * 1024);
 	for (i = 0; i < 4096; i++)
@@ -735,6 +736,8 @@ void aw_fel_write_and_execute_spl(libusb_device_handle *usb,
 		exit(1);
 	}
 
+	/* re-enable the MMU if it was enabled by BROM */
+	if(tt != NULL)
 		aw_restore_and_enable_mmu(usb, tt);
 }