mt76: make const array 'data' static, shrinks object size

Don't populate the const array 'data' on the stack but instead
make it static. Makes the object code smaller by 78 bytes:

Before:
   text    data     bss     dec     hex filename
   5438    1080       0    6518    1976 mediatek/mt76/mt76x2/usb_mcu.o

After:
   text    data     bss     dec     hex filename
   5296    1144       0    6440    1928 mediatek/mt76/mt76x2/usb_mcu.o

(gcc version 8.2.0 x86_64)

Signed-off-by: Colin Ian King <colin.king@canonical.com>
Signed-off-by: Felix Fietkau <nbd@nbd.name>