s2io.c: Standardize statistics accessors

Regularize the declaration and uses of
	struct config_param *config = &sp->config;
	struct mac_info *mac_control = &sp->mac_control;
and use
	struct stat_block *stats = mac_control->stats_info;
	struct swStat *swstats = &stats->sw_stat;
	struct xpakStat *xstats = &stats->xpak_stat;
and convert the longish uses like
	nic->mac_control.stats_info->sw_stat.<foo>
to
	swstats-><foo>
etc.

This also makes the statistics code marginally smaller
and presumably faster.

Old:
$ size s2io.o
   text	   data	    bss	    dec	    hex	filename
 114289	    516	  33360	 148165	  242c5	s2io.o
New:
$ size s2io.o
   text	   data	    bss	    dec	    hex	filename
 114097	    516	  33360	 147973	  24205	s2io.o
Signed-off-by: Joe Perches <joe@perches.com>
Acked-by: Sreenivasa Honnur <sreenivasa.honnur@neterion.com>
Signed-off-by: David S. Miller <davem@davemloft.net>