usb: gadget: ncm: make global variable ndp*_opts read only

There is ndp16_opts and ndp32_opts which are both global and only one
member is ever written. This patch makes the variable read-only and
moves the one member that is ever written into the private struct f_ncm.

Signed-off-by: Sebastian Andrzej Siewior <bigeasy@linutronix.de>
Signed-off-by: Felipe Balbi <balbi@ti.com>