Corrections in serial_open and serial_set_timeout

ftdi_context->timeout is the timeout used while libusb_bulk_transfer.
On the other hand, libdivecomputer context is used internally by
libdivecomputer. So inappropriate to set both here. By default libftdi
sets a timeout of 5000 ms on both read and write.

Usage of struct ftdi_context was wrong. Fixed it.

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