usb: ch9.h: usb_endpoint_maxp() uses __le16_to_cpu()

The usb/ch9.h will be installed to /usr/include/linux,
and be used from user space.
But le16_to_cpu() is only defined for kernel code.
Without this patch, user space compile will be broken.
Special thanks to Stefan Becker

Reported-by: Stefan Becker <chemobejk@gmail.com>
Signed-off-by: Kuninori Morimoto <kuninori.morimoto.gx@renesas.com>
Cc: stable <stable@vger.kernel.org>
Signed-off-by: Greg Kroah-Hartman <gregkh@suse.de>