USB: serial: ftdi_sio: use cur_altsetting for consistency

ftdi_determine_type() function had this construct in it to get the
number of the interface it is operating on:

  inter = serial->interface->altsetting->desc.bInterfaceNumber;

Elsewhere in this driver cur_altsetting is used instead for this
purpose.  Change ftdi_determine_type() to use cur_altsetting
for consistency.

Signed-off-by: Mychaela N. Falconia <falcon@freecalypso.org>
[ johan: fix old style issues; drop braces and random white space ]
Signed-off-by: Johan Hovold <johan@kernel.org>