usb: gadget: do not use __constant_cpu_to_le16

A trivial patch.

cpu_to_le16() is capable enough to detect __builtin_constant_p()
and to use an appropriate compile time ___constant_swahbXX()
function.

So we can use cpu_to_le16() instead of __constant_cpu_to_le16().

Signed-off-by: Sergey Senozhatsky <sergey.senozhatsky@gmail.com>
Signed-off-by: Felipe Balbi <felipe.balbi@linux.intel.com>