brcmsmac: use container_of instead of cast

Now "struct si_pub pub" does not have to be the first member in struct
si_info any more, if it is the resulting code after compilation should
be the same.

Acked-by: Arend van Spriel <arend@broadcom.com>
Signed-off-by: Hauke Mehrtens <hauke@hauke-m.de>
Signed-off-by: John W. Linville <linville@tuxdriver.com>