Added usb_open_dev_fd for opening on android

Open using provided file descriptor. This function was
earlier left out.
One crucial point is not to close the USB device
( which is done normally by libftdi ).
Nor try to open it again. It is already open.

Signed-off-by: Venkatesh Shukla <venkatesh.shukla.eee11@iitbhu.ac.in>