staging: comedi: usbdux: fix sparse endianness warnings

Sparse has many warnings like:

drivers/staging/comedi/drivers/usbdux.c:366:32: warning: cast to
restricted __le16

on lines on which devpriv->in_buf or devpriv->insn_buf are passed to
le16_to_cpu().  This suggests that both of these variables should actually
be of type __le16.

Signed-off-by: Chase Southwood <chase.southwood@gmail.com>
Reviewed-by: Ian Abbott <abbotti@mev.co.uk>
Cc: H Hartley Sweeten <hsweeten@visionengravers.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>