ARC: remove redundant READ_ONCE() in cmpxchg loop

This patch reverts commit 7082a29c ("ARC: use ACCESS_ONCE in cmpxchg
loop").

It is not necessary to use READ_ONCE() because cmpxchg contains barrier. We
can get it from commit d57f7272

 ("ARC: add compiler barrier to LLSC
based cmpxchg").

Signed-off-by: Bang Li <libang.linuxer@gmail.com>
Signed-off-by: Vineet Gupta <vgupta@kernel.org>