usb: gadget: f_mass_storage: change default value of the removable parameter

This commit changes the default value of the removable module parameter
from “y” to “n”.  This comes with line with file_storag's default and
seems to be a better default.

Signed-off-by: Michal Nazarewicz <mina86@mina86.com>
Signed-off-by: Felipe Balbi <balbi@ti.com>