USB: fix usb-serial/ftdi build warning

Fix annoying build warning:

drivers/usb/serial/ftdi_sio.c:890: warning: enumeration value `FT232RL' not handled in switch

Also add logic to detect FT232R chips (version 6.00, usb 2.0 full speed),
so that case isn't completely useless.  (NOTE:  FT232RL and FT232RQ are
the same chip in different packages:  L is SSOP, Q is QFN.)

Signed-off-by: David Brownell <dbrownell@users.sourceforge.net>
Signed-off-by: Greg Kroah-Hartman <gregkh@suse.de>