parisc: avoid unitialized variable warning in pa_memcpy()

Avoid this warning, while still prevent gcc from optimizing away the exception code:
arch/parisc/lib/memcpy.c: In function ‘pa_memcpy’:
arch/parisc/lib/memcpy.c:256:2: warning: ‘dummy’ may be used uninitialized in this function [-Wuninitialized]

Signed-off-by: Helge Deller <deller@gmx.de>