rndis_wlan: Use static const

Using static const generally increases object text and decreases data size.
It also generally decreases overall object size.

   text	   data	    bss	    dec	    hex	filename
  41757	   2205	   9896	  53858	   d262	drivers/net/wireless/rndis_wlan.o.old
  41653	   2205	   9880	  53738	   d1ea	drivers/net/wireless/rndis_wlan.o.new

Changed functions rndis_set_oid and set_bssid to take const *'s.

Signed-off-by: Joe Perches <joe@perches.com>
Signed-off-by: John W. Linville <linville@tuxdriver.com>