asm-generic: Fix 32 bit __generic_cmpxchg_local

Commit 656e9007 ("asm-generic: avoid __generic_cmpxchg_local
warnings") introduced a typo that means the code is incorrect for 32 bit
values. It will work fine for postive numbers, but will fail for
negative numbers on a system where longs are 64 bit.

Fixes: 656e9007 ("asm-generic: avoid __generic_cmpxchg_local warnings")
Signed-off-by: David McKay <david.mckay@codasip.com>
Signed-off-by: Stuart Menefy <stuart.menefy@codasip.com>
Signed-off-by: Arnd Bergmann <arnd@arndb.de>