usb: gadget: hid: consistently use 2^n - 1 for max values

A maximum value which fits in 16 bits, unsigned, is 65535.

Acked-by: Geert Uytterhoeven <geert@linux-m68k.org>
Signed-off-by: Andrzej Pietrasiewicz <andrzej.p@samsung.com>
Signed-off-by: Felipe Balbi <balbi@ti.com>