staging: most: net: make array 'broadcast' static, shrinks object size

Don't populate the array 'broadcast' on the stack but instead make it
static. Makes the object code smaller by 53 bytes:

Before:
   text	   data	    bss	    dec	    hex	filename
  10511	   1521	    448	  12480	   30c0	drivers/staging/most/net/net.o

After:
   text	   data	    bss	    dec	    hex	filename
  10394	   1585	    448	  12427	   308b	drivers/staging/most/net/net.o

(gcc version 8.2.0 x86_64)

Signed-off-by: Colin Ian King <colin.king@canonical.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>