USB: serial: ftdi_sio: convert to use dev_groups

The driver core supports the ability to handle the creation and removal
of device-specific sysfs files in a race-free manner.

Signed-off-by: Jiasheng Jiang <jiasheng@iscas.ac.cn>
[ johan: rebase on type rework, make groups static, clean up, amend
         commit message ]
Signed-off-by: Johan Hovold <johan@kernel.org>