brcmsmac: Remove unused macro SI_MSG

commit 6236dc2e ("brcmsmac: remove some redundant chip common workarounds")
removed the last uses.

Signed-off-by: Joe Perches <joe@perches.com>
Acked-by: Arend van Spriel <arend@broadcom.com>
Signed-off-by: John W. Linville <linville@tuxdriver.com>