USB: serial: ftdi_sio: clean up attribute handling

The driver exposes two attributes for all chip types but FT232A, which
doesn't have a configurable latency timer, and SIO, which (probably)
doesn't support the event-char mechanism either.

Explicitly test for the exceptions rather than list each and every
supported device type in the attribute helpers.

Signed-off-by: Johan Hovold <johan@kernel.org>