USB: serial: metro-usb: get data from device in Uni-Directional mode.

We should send special control command to tell device start or stop
transmitting a data.

In Bi-Directional mode that cmd`s are not required.

Signed-off-by: Aleksey Babahin <tamerlan311@gmail.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>