x86: fix percpu_{to,from}_op()

- the byte operand constraints were wrong for 32-bit
- the to-op's input operands weren't properly parenthesized

[ Impact: fix possible miscompilation or build failure ]

Signed-off-by: Jan Beulich <jbeulich@novell.com>
Signed-off-by: H. Peter Anvin <hpa@zytor.com>