x86/xen: clean up clang build warning

In the case where sizeof(maddr) != sizeof(long) p is initialized and
never read and clang throws a warning on this.  Move declaration of
p to clean up the clang build warning:

warning: Value stored to 'p' during its initialization is never read

Signed-off-by: Colin Ian King <colin.king@canonical.com>
Reviewed-by: Juergen Gross <jgross@suse.com>
Signed-off-by: Boris Ostrovsky <boris.ostrovsky@oracle.com>