atm: zatm: Fix empty body Clang warnings

Clang warns:

drivers/atm/zatm.c:513:7: error: while loop has empty body
[-Werror,-Wempty-body]
        zwait;
             ^
drivers/atm/zatm.c:513:7: note: put the semicolon on a separate line to
silence this warning

Get rid of this warning by using an empty do-while loop. While we're at
it, add parentheses to make it clear that this is a function-like macro.

Link: https://github.com/ClangBuiltLinux/linux/issues/42


Suggested-by: Masahiro Yamada <yamada.masahiro@socionext.com>
Signed-off-by: Nathan Chancellor <natechancellor@gmail.com>
Signed-off-by: David S. Miller <davem@davemloft.net>