xen/arm: gic-v3: Add missing breaks gicv3_read_apr()

Commit 78e67c99eb "arm/gic: Get rid of READ/WRITE_SYSREG32"
mistakenly converted all the cases in gicv3_read_apr() to fall-through.

Rather than re-instating a return per case, add the missing break and
keep a single return at the end of the fucntion.

Fixes: 78e67c99eb ("arm/gic: Get rid of READ/WRITE_SYSREG32")
Signed-off-by: Julien Grall <jgrall@amazon.com>
Reviewed-by: Stefano Stabellini <sstabellini@kernel.org>

xen/arch/arm/gic-v3.c

@@ -1167,12 +1167,15 @@ static unsigned int gicv3_read_apr(int apr_reg)
     case 0:
         ASSERT(gicv3.nr_priorities > 4 && gicv3.nr_priorities < 8);
         apr = READ_SYSREG(ICH_AP1R0_EL2);
+        break;
     case 1:
         ASSERT(gicv3.nr_priorities > 5 && gicv3.nr_priorities < 8);
         apr = READ_SYSREG(ICH_AP1R1_EL2);
+        break;
     case 2:
         ASSERT(gicv3.nr_priorities > 6 && gicv3.nr_priorities < 8);
         apr = READ_SYSREG(ICH_AP1R2_EL2);
+        break;
     default:
         BUG();
     }