efi: address violation of MISRA C Rule 16.3

Use agreed syntax for pseudo-keyword fallthrough to meet the
requirements to deviate 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: Marek Marczykowski-Górecki <marmarek@invisiblethingslab.com>

xen/common/efi/runtime.c

@@ -699,7 +699,7 @@ int efi_runtime_call(struct xenpf_efi_runtime_call *op)
 
         if ( !efi_enabled(EFI_RS) || (efi_rs->Hdr.Revision >> 16) < 2 )
             return -EOPNOTSUPP;
-        /* XXX fall through for now */
+        fallthrough; /* XXX fall through for now */
     default:
         return -ENOSYS;
     }