staging: comedi: drivers: replace le16_to_cpu() with usb_endpoint_maxp()

Use macro introduced in commit 939f325f
("usb: add usb_endpoint_maxp() macro")

Signed-off-by: Cheah Kok Cheong <thrust73@gmail.com>
Reviewed-by: Ian Abbott <abbotti@mev.co.uk>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>