USB: ftdi_sio: Replace BKL with a mutex

As Alan Cox have pinpointed the driver still required protection against
parallels calls to the config ioctl(). If lock is still necessary the use of
BKL is abused here. So replace BKL with a more convenient mutex.

Signed-off-by: Alessio Igor Bogani <abogani@texware.it>
Cc: Greg Kroah-Hartman <gregkh@suse.de>
Cc: Johan Hovold <jhovold@gmail.com>
Cc: Alan Cox <alan@linux.intel.com>
Cc: Daniel Mack <daniel@caiaq.de>
Cc: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Greg Kroah-Hartman <gregkh@suse.de>