netfilter: ebt_stp: Use generic functions for comparisons

Instead of unnecessary const declarations, use the generic functions to
save a little object space.

$ size net/bridge/netfilter/ebt_stp.o*
   text	   data	    bss	    dec	    hex	filename
   1250	    144	      0	   1394	    572	net/bridge/netfilter/ebt_stp.o.new
   1344	    144	      0	   1488	    5d0	net/bridge/netfilter/ebt_stp.o.old

Signed-off-by: Joe Perches <joe@perches.com>
Signed-off-by: Pablo Neira Ayuso <pablo@netfilter.org>