i2c: create debugfs entry per adapter

Two drivers already implement custom debugfs handling for their
i2c_adapter and more will come. So, let the core create a debugfs
directory per adapter and pass that to drivers for their debugfs files.

Signed-off-by: Wolfram Sang <wsa+renesas@sang-engineering.com>
Signed-off-by: Wolfram Sang <wsa@kernel.org>