USB: spcp8x5: add proper modem-status support

Fetch modem status on carrier_raised and tiocmget.

This driver appeared to support modem-status but only read the modem
status registers once at open and then used that cached value for all
further enquires.

Signed-off-by: Johan Hovold <jhovold@gmail.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>