USB: ftdi_sio: fix null deref at port probe

Fix NULL-pointer dereference when probing an interface with no
endpoints.

These devices have two bulk endpoints per interface, but this avoids
crashing the kernel if a user forces a non-FTDI device to be probed.

Note that the iterator variable was made unsigned in order to avoid
a maybe-uninitialized compiler warning for ep_desc after the loop.

Fixes: 895f28ba ("USB: ftdi_sio: fix hi-speed device packet size
calculation")

Reported-by: Mike Remski <mremski@mutualink.net>
Tested-by: Mike Remski <mremski@mutualink.net>
Cc: <stable@vger.kernel.org>	# 2.6.31
Signed-off-by: Johan Hovold <johan@kernel.org>