arm64: atomics: Remove '&' from '+&' asm constraint in lse atomics

The lse implementation of atomic64_dec_if_positive uses the '+&' constraint,
but the '&' is redundant and confusing in this case, since early clobber
on a read/write operand is a strange concept.

Replace the constraint with '+'.

Signed-off-by: Will Deacon <will.deacon@arm.com>