aarch64: Fix assembling chacha20-aarch64.S with clang/llvm

* cipher/chacha20-aarch64.S: Remove superfluous lane counts.

When referring to a specific lane, one doesn't need to specify
the total number of lanes of the register. With GNU binutils,
both forms are accepted, while clang/llvm rejects the form
with the unnecessary number of lanes.

• Signed-off-by: Martin Storsjö <martin@martin.st>