sparc64: fix build with older gccs

Fix errors missed in 2065061e:
 CC    sparc64-softmmu/helper.o
cc1: warnings being treated as errors
/src/qemu/target-sparc/helper.c: In function 'get_physical_address':
/src/qemu/target-sparc/helper.c:426: warning: 'context' may be used uninitialized in this function
/src/qemu/target-sparc/helper.c:426: note: 'context' was declared here
Signed-off-by: NBlue Swirl <blauwirbel@gmail.com>

target-sparc/helper.c

@@ -444,6 +444,7 @@ static int get_physical_address_data(CPUState *env,
         context = env->dmmu.mmu_secondary_context & 0x1fff;
         break;
     case MMU_NUCLEUS_IDX:
+    default:
         context = 0;
         break;
     }