Implementation of serial_enumerate

Proper implementation of serial_enumerate.c
Get a list of attached ftdi devices and pass the struct libusb_device in
callback function for every device found. For this, the definition of
serial_callback_t has been changed. Changing const char *name to const void
*params so that char as well as libusb_device can be passed.

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