staging: comedi: usbdux: fix more sparse endianness warnings

Sparse shows a couple of warnings like:

drivers/staging/comedi/drivers/usbdux.c:889:20: warning: incorrect type in assignment (different base types)
drivers/staging/comedi/drivers/usbdux.c:889:20:    expected unsigned short [unsigned] [short] [usertype] <noident>
drivers/staging/comedi/drivers/usbdux.c:889:20:    got restricted __le16 [usertype] <noident>

This is the result of a couple of calls to cpu_to_le16() being assigned to
uint16_t typed variables.  Switch the types of these variables/pointers to
__le16 accordingly.

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