i2c: bail out early when RDWR parameters are wrong

The buggy parameters currently get caught later, but emit a noisy WARN.
Userspace should not be able to trigger this, so add similar checks much
earlier. Also avoids some unneeded code paths, of course. Apply kernel
coding stlye to a comment while here.

Reported-by:  <syzbot+ffb0b3ffa6cfbc7d7b3f@syzkaller.appspotmail.com>
Tested-by:  <syzbot+ffb0b3ffa6cfbc7d7b3f@syzkaller.appspotmail.com>
Signed-off-by: Wolfram Sang <wsa+renesas@sang-engineering.com>
Signed-off-by: Wolfram Sang <wsa@kernel.org>