x86/psr: address violation of MISRA C Rule 16.3

Address a violation of MISRA C:2012 Rule 16.3:
"An unconditional `break' statement shall terminate every
switch-clause".

No functional change.

Signed-off-by: Federico Serafini <federico.serafini@bugseng.com>
Acked-by: Jan Beulich <jbeulich@suse.com>

xen/arch/x86/psr.c

@@ -276,6 +276,7 @@ static enum psr_feat_type psr_type_to_feat_type(enum psr_type type)
 
     default:
         ASSERT_UNREACHABLE();
+        break;
     }
 
     return feat_type;