sparc64: Fix constraints on swab helpers.

[ Upstream commit 5a2b59d3 ]

We are reading the memory location, so we have to have a memory
constraint in there purely for the sake of showing the data flow
to the compiler.

Reported-by: Martin K. Petersen <martin.petersen@oracle.com>
Signed-off-by: David S. Miller <davem@davemloft.net>
Signed-off-by: Jiri Slaby <jslaby@suse.cz>