parisc: optimize mtsp(0,sr) inline assembly

commit 92b59929825d67db575043a76651865d16873b36 upstream.

If the value which should be moved into a space register is zero, we can
optimize the inline assembly to become "mtsp %r0,%srX".

Signed-off-by: Helge Deller <deller@gmx.de>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>