Staging: wlan-ng: convert usb_prism_tbl[] array into a const array

This patch convert usb_prism_tbl[] into a const array
since it is not modified anywhere in prism2usb.c file.

Signed-off-by: Claudiu Beznea <claudiu.beznea@gmail.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>