tty: usb_serial_mos7720: Fix get_lsr_info

I made a correction for get_lsr_info, now it returns some meaningful
information. I tested it with two simultaneous simplex modem channels.

it is attached

Signed-off-by: Kees Schoenmakers <k.schoenmakers@sigmae.nl>
Signed-off-by: Alan Cox <alan@linux.intel.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@suse.de>